I © Copyright 2019 Braden A. Zahora

Synthesis and Reactivity of PtII Complexes with Secondary Sphere N-H Moieties

Braden A. Zahora

A dissertation submitted in partial fulfillment of the requirements for the degree of

Doctor of Philosophy

University of Washington 2019

Reading Committee: Karen I. Goldberg, Chair D. Michael Heinekey Forrest E. Michael

Program Authorized to Offer Degree: Department of Chemistry

University of Washington

Abstract

Synthesis and Reactivity of PtII Complexes with Secondary Sphere N-H Moieties

Braden A. Zahora

Chairperson of the Supervisory Committee: Professor Karen I. Goldberg Department of Chemistry

Direct transformations of abundant hydrocarbons into higher valued products would circumvent energy intensive processes, reforming the chemical industry. Late transition metals, particularly platinum, are known to selectively convert alkanes into functionalized products.

However, a practical method with an economically viable oxidant has yet to be discovered. New methods for alkane activation may yield further advances towards this goal. This dissertation focuses on how platinum methyl complexes with ligand-based protons can form methane, the microscopic reverse of methane activation. Chapter 1 provides an introduction on the need to develop new methods to functionalize hydrocarbons. Furthermore, a survey of the literature with respect to how late metal complexes can undergo productive reactivity with substrates which are deemed necessary for practical alkane partial oxidation is presented.

Chapter 2 focuses on the synthesis and reactivity of 5-(6-methyl-2-pyridyl)-3-tert- butylpyrazolate (NNMe) and 2-(5-tert-butylpyrazol-3-yl)-6 (diethylaminomethyl)pyridine (NNNEt) ligated PtII complexes. Once formed, conditions necessary for methane formation were found and

iii it was determined that the ligand-based N-H was not involved in methane formation. Chapter 3 explores synthesis and reactivity of 5-tert-butyl-1,3-bis(pyrazol-3-yl)pyridine (NNN) and 5-tert- butyl-1,3-bis(pyrazol-3-yl)benzene (NCN) ligated PtII-alkyl complexes with electrophilic reagents.

It was found that tridentate ligated pyrazolate PtII-R systems can undergo protonation at the ligand if there is a weak pyridine trans donor (vs strong phenyl trans donor) to the PtII-R moiety. However, reactivity with methyl iodide occurred at the metal first, with further ligand methylation spectroscopically observed for the NNN ligated Pt-CH3 complex. Chapter 4 discusses the synthesis and characterization of bis(phosphino)amine ligated PtII complexes, which contained a ligand-based

N-H. Taking advantage of unfavorable steric congestion of bulky Ph groups on the ligand, methane formation was observed from thermolysis reactions of a bis(phosphino)amine ligated Pt(CH3)2.

Although additional characterization of the resulting metal-containing product is required, it appears that methane was formed through cooperation of the ligand-based N-H moiety and the Pt-CH3 ligand.

Table of Contents

List of Figures ...... ix List of Schemes ...... xiii List of Tables ...... xiv Glossary ...... xv Compound Numbering Scheme ...... xvii Acknowledgements ...... xxii Chapter Contributions ...... xxiv Chapter 1 Introduction...... 1 1.1 A Need to Reduce Hydrocarbon Flaring ...... 1 1.2 Homogeneous Partial Oxidation of Methane, an Overview ...... 4 1.3 Designing A New System for Alkane Functionalization ...... 8 1.4 Dissertation Summary ...... 14 1.5 Notes for Chapter 1 ...... 16 Chapter 2 Synthesis and Reactivity of Bidentate and Hemilabile Pyrazolate Ligated Pt Complexes ...... 22 2.1 Introduction ...... 22 2.2 Results and Discussion ...... 28 2.21 Synthesis of Bidentate Ligand Supported Pyrazolate PtII-Complexes ...... 28 2.22 Preparation of (NNN)Et Ligated PtII Complexes ...... 31 2.23 The Reactivity of B2a/B2b and 4a with Acid...... 32 2.24 Release of methane from B4a under acidic conditions ...... 35

2.25 Release of methane from B7 and B4a by thermolysis ...... 36 2.26 Reactivity of Pt(*NNN)EtX Under Basic Conditions and in the Presence of Exogeneous Ligands ...... 41 2.3 Conclusion ...... 45 2.4 Experimental ...... 46 2.41 General Experimental ...... 46 2.42 Synthesis, Characterization and Spectroscopic Data ...... 48 2.43 X-ray Crystallography General Information ...... 71

2.5 Notes to Chapter 2 ...... 73 Chapter 3 Synthesis of Pyrazolate Supported Tridentate PtII Alkyl Complexes and Reactivity with Electrophiles ...... 77 3.1 Introduction ...... 77 3.2 Results and Discussion ...... 82 3.21 Preparation of HNNNH Ligated Complexes ...... 82 3.22 Preparation of HNCNH Ligated Complexes ...... 84 3.23 Synthesis of *NNN* Ligated Pt-Alkyl Species ...... 88 3.24 Synthesis of *NCN* Ligated Pt-Alkyl Species ...... 92 + + II 3.25 Electrophile (H and CH3 ) Addition to Pyrazolate Supported Pt -alkyl Compounds . 94 3.25.1 Proton Addition to (*NNN*) ligated PtII-alkyl (C6, C7, C8) Compounds ...... 95

3.25.2 Proton Addition to Pt(*NCN*)CH3 (C11, C12) and Pt(*NCN*)C6H5 (C13) ...... 101 II 3.25.3 Reactivity of *NCN* ligated (C11) and *NNN* ligated (C8) Pt -CH3 complexes with CH3I ...... 103 3.3 Conclusion ...... 106 3.4 Experimental and NMR Data ...... 108 3.41 General Experimental ...... 108 3.42 Synthesis, Characterization and Spectroscopic Data ...... 109 3.43 X-ray Crystallography General Information ...... 136 3.5 References to Chapter 3 ...... 140 Chapter 4 Synthesis of Bis(phosphino)amine Ligated PtII Species and Investigations Towards C-H coupling ...... 144 4.1 Introduction ...... 144 4.2 Results and Discussion ...... 148 4.21 Ligation of Protic Amino(bisphosphines) to PtII ...... 148

4.22 Reactivity of Pt(*N(P(C6H5)2)2)2 (D3) towards X-H Activation...... 153 H H t 4.23 Towards C-H coupling from Pt( N(P(C6H5)2)2(CH3)2 (D1a) and Pt( N(P( Bu)2)2(CH3)2 (D1b) ...... 154 4.3 Conclusion ...... 162 4.4 Experimental ...... 163 4.41 General Experimental ...... 163 4.42 Synthesis, Characterization and Spectroscopic Data ...... 164 4.43 X-ray Crystallography General Information ...... 176

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4.5 Notes to Chapter 4 ...... 180 Bibliography ...... 185

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List of Figures

Figure 1.0 1 Image of light pollution across the central United States...... 2 Figure 1.0 2 Catalytic system developed by the Shilov transforming methane to methanol (and chloromethane)...... 5

Figure 1.0 3 Proposed catalytic cycle for methane functionalization using O2...... 8 Figure 1.0 4 Activation of R-H substrate by an intermolecular (top) or intramolecular (bottom) pathway...... 11 Figure 1.0 5 Activation of R-H substrate by an (a) electrophilic aromatic substitution pathway, and (b) concerted metalation deprotonation pathway...... 12 I I Figure 1.0 6 Activation of C6H6 substrate by (a) Ir /Rh (PNP) complexes, and by a (b) RhI(PNNNP) complex...... 13

Figure 2.0 1 Ligand examples employed which have allowed isolation of PtIV methylhydrides ..23 Figure 2.0 2 (a) Metalation of HNNH (b) Metalation of HNNMe ...... 28 Figure 2.0 3 Metalation of HNNNEt to PtII to form B4a, B4b and B5...... 31

Figure 2.0 4 (a)Protonation of B2a/B2b with HBF4 etherate to form either B6a or B6b and (b). Protonation of B2a/B2b with HCl etherate to form methane after 2 added equiv...... 33 Et Figure 2.0 5 Protonation of Pt(*NNN) CH3 (B4a) with HBF4 etherate to form H Et [Pt( NNN) CH3][BF4] (B7). Further Protonation of B7 with HCl etherate to form H Et [Pt( NNN) Cl][BF4] (B5a)...... 35 Figure 2.0 6 Potential mechanism of acetonitrile hydrolysis...... 37

Figure 2.0 7 a) Thermolysis of B7 in C6D6 to form multiple Pt. b) Thermolysis of B4a in C6H6 to form B4b and multiple Pt...... 38 2 Figure 2.0 8 H NMR spectrum (107 MHz) in C6H6 of the thermolysis of B1 in C6D6.

Referenced to additional CD3CN at 1.94 ppm. Deuterated methylene deuterons at 2.7 - 3.1

ppm. Aryl Pt-C6D5 deuterons at 6.8 – 7.6 ppm...... 39

Figure 2.0 9 Potential mechanism for the thermolysis of B4a in C6D6 to form CH4 and Pt-C6D5...... 40

t Figure 2. 10 Reaction of B5 with KO Bu in C6D6 to form B5b in situ...... 41

Figure 2.1 1 (a) Proposed reaction resulting in disappearance of methylene proton resonance in 1 1 the H NMR spectrum by deuteration. (b). H NMR spectrum of B4a in CD3CN (bottom) and after addition of 2 equiv. of NaOtBu (top)...... 43 Figure 2.1 2 Reaction of B4a with exogeneous L ligands and how methylene protons change in the 1H NMR spectrum ...... 44 Figure 2.1 3-2.3 6 Spectroscopic Data for Chapter 2………………………………………………...49

Figure 3.0 1 (a) Reaction of Pt(DMEP/DMPP)(CH3)2 with HCl. (b) Reaction of Pt(bipym)(CH3)2

with CH3I. (c) Computational predicted products for reaction of Pt(cbipy)(CH3)2 with CH3I...... 80 Figure 3.0 2 Metalation of HNNNH to PtII to form [Pt(HNNNH)Cl][Cl] (C1) and subsequent H H H H reactions to form [Pt( NNN )Cl][BF4] (C2), and [Pt( NNN )NCCH3][(BF4)2] (C3)...... 83 H H II H H R’ Figure 3.0 3 Metalation of NCN to Pt to form Pt( NCN ) Cl (C4) and reactivity with AgBF4 to form C4b...... 85 H H Figure 3.0 4 Thermal ellipsoid plots (50% probability) of Pt4(*NCN*)( NCN )3...... 87

Figure 3.0 5 Reactivity of C1 with CH3Li to form [Li2Cl][Pt(*NNN*)CH3] (C6) and in the

presence of CH3CN to form [Li(THF)]2[Pt(*NNN*)CH2CN]2 (C7). Cation exchange

reaction of C6 with PPNCl to form [PPN][Pt(*NNN*)CH3 (C8). Thermal ellipsoid plots (50 % probability) of C6, C7 and C8...... 89 H H t Figure 3.0 6 Reactivity of (a) Pt( NCN )Cl (R = H, C4 and R = Bu, C5) with CH3Li to form R t [Li]2[Pt(*NCN*) CH3] (R = H, C11 and R = Bu, C12) and (b) C4 with C6H5Li to R form[Li]2[Pt(*NCN*) C6H5], C13...... 93

Figure 3.0 7 Protonation of (a) C7 with HCl and HBF4 etherate/2,6 dimethoxy pyridinium H H H H tetrafluoroborate to form [Pt( NNN )(CH2CN)][Cl] (C7a) and [Pt( NNN )(CH2CN)][BF4]

(C7b), respectively. (b) Speciation during the protonation of [Li2Cl][Pt(*NNN*)CH3 (C6), H H H H and formation of complexes: [Pt( NNN )CH3][Cl] (C9), [Pt( NNN )Cl][Cl] (C1), and H H [Pt( NNN )Cl][BF4] (C2) in acetone-d6. CDH3 formation most likely proceeds through N-

H/D exchange with solvent. No H/D exchange occurs in CD3CN. (c) Speciation during the H H H H protonation of C8: [Pt( NNN )CH3][Cl] (C9), [Pt( NNN )CH3][BF4] (C14), H H H H [Pt( NNN )Cl][Cl] (C1), and [Pt( NNN )NCCD3][(BF4)2] (C3)...... 97

Figure 3.0 8 1H NMR spectra (500 MHz) showing the addition of 2,6-dimethoxypyridinium

tetrafluoroborate to a solution of (a) [Li2Cl][Pt(*NNN*)CH3] (C6) in acetone-d6 and (b)

[PPN][Pt(*NNN*)CH3] (C8) in CD3CN. In the spectra above, the aryl region is highlighted...... 99

Figure 3.0 9 Speciation during the protonation of [Li2][Pt(*NCN*)R] (R = CH3: [R’ = H (C11), t t R’ = Bu (C12)]; R = C6H5 [R’ = Bu (C13)]) and formation of complexes: R - t H H R’ - t [Li2][Pt(*NCN*) L] (L = Cl : R’ = Bu (C5a), H (C4a)), Pt( NCN ) L (L = Cl [R’ = Bu

(C5)]); L = unknown [R’ = H]) in THF-d8...... 101

Figure 3. 10 VT 1H NMR spectrum (500 MHz) of addition of HCl etherate (1M, 0.016 mmol) to a solution of C12 at -73 °C...... 102

Figure 3.1 1 Reaction of C11 ([Li(THF)2][Pt(*NCN*)CH3]) with 2 equiv. CH3I to form a IV tBu mixture of two Pt -(CH3)2 complexes, proposed to be [Li(THF)2]2[Pt(*NCN*) (CH3)2I] tBu and a potential 5 coordinate [Li2(THF)4I][Pt(*NCN*) (CH3)2]. These two Pt compounds

decompose to form ethane, CH4, CH3D, and unknown Pt complexes...... 104

Figure 3.1 2 Reaction of C6 ([Li2Cl][Pt(*NNN*)CH3]) and C8 ([PPN][Pt(*NNN*)CH3]) with 1 IV equiv. CH3I to form a mixture of two Pt -(CH3) complexes. Addition of KI to the [Pt] H - mixture to form proposed [Pt(*NNN*) (CH3)2I] . Reaction of C8 with 2 equiv. CH3I to CH3 H form [Pt( NNN*) (CH3)2I][I] (C15)...... 105 Figure 3.13-3.44 Spectroscopic Data for Chapter 3………………………………………………...110 Figure 4.0 1 Proposed acetophenone hydrogenation by (a) Noyori and (b) Gordon...... 145 Figure 4.0 2 (a) Proposed mechanism of isomerization of 1-pentene to trans 2-pentene by MLC. H (b) Heterolytic hydrogen cleavage by Fe(*N(P(C6H5)2)2)2( N(P(C6H5)2)2) to form H Fe( N(P(C6H5)2)2)2(H)2 by MLC...... 146 H t II H Figure 4.0 3 Metalation of N(PR2)2 (R = C6H5, Bu) with Pt to form Pt( N(P(R)2)(CH3)2 [R = t H t C6H5 (D1a), Bu (D1b)] and Pt( N(P( Bu)2)2)(Cl)2 (D4)...... 149 t Figure 4.0 4 Synthetic pathway resulting in formation of Pt(*N(PR2)2)2 [R = C6H5 (D3), Bu]. H Homoleptic [Pt( N(P(C6H5)2)2)2][BF4] (D2) is an intermediate in the formation of D3.. ...151 1 Figure 4.0 5 H NMR spectra of D3 (bottom) and subsequent pressurization with H2 (middle) or

addition of H2O (top) and generation of triplet at -0.18 ppm (in middle and top spectra). .153

Figure 4.0 6 (a) Thermolysis of D1a in C6D6 to form a paramagnetic species. (b) Addition of H HBF4 etherate to D1a in pyridine-d5 to form [Pt( N(P(C6H5)2)2)(CH3)(pyr-d5)][BF4] (D5)

and subsequent thermolysis to generate ethane. (c) Thermolysis of D1a in C6D6 with 2

equiv. P(C6H5)3 to form Pt(P(C6H5)3)2(CH3)2 and D3. (d) Thermolysis of D1a to form Pt2(µ-

*N(P(C6H5)2)2)(µ-CH2)(pyr)2 (D6) and a paramagnetic product...... 157 Figure 4.0 7 Thermal ellipsoid plot of D6 at 50 % probability and H-atoms omitted for clarity. Right orientation highlights “A-Frame”. Selected bond lengths and angles for D6: Pt(1)- P(1) 2.299(1) Å, Pt(1)-C(59) 2.070(4) Å, Pt(1)-N(3) 2.153(4) Å, Pt(1)-Pt(2) 2.9701(6) Å. 159 Figure 4.08-4.15 Spectroscopic Data for Chapter 4……………………………………….………..165

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List of Schemes

Scheme 1.01. Catalytica reaction ...... 6

III II Scheme 1.02. Heterolytic cleavage of H2 by an a) Ir complex and b) Pt complex ...... 10

Scheme 2.01 (a) Protonation of pyridinophane ligated PtII (b) Protonation of DMEP(n=1) /DMPP(n=2) ligated PtII ...... 25 II Scheme 3.01. Generalized reaction of L2Pt (CH3)2 with HX and CH3X ...... 78

+ Scheme 3.02. Metalation plan to form Pt-CH3 complexes. Reactivity of Pt-CH3 with EX (E = CH3 or H+)...... 81 Scheme 4.01. Elimination of methane upon thermolysis in the (a) presence of pyridine (b) absence of pyridine ...... 147

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List of Tables

Table 2.1 Parameters for X-ray Structures in Chapter 2 ...... 72

Table 3.1 Parameters for X-ray Structures in Chapter 3 ...... 137

Table 3.2 Parameters for X-ray Structures in Chapter 3 ...... 138

Table 3.3 Parameters for X-ray Structures in Chapter 3 ...... 139

Table 4.1 Parameters for X-ray Structures in Chapter 4 ...... 178

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Glossary

Å angstrom Ac acetate Ar aryl BDE bond dissociation energy bdmimp 3,5-bis(2,6-dimethylphenyliminoacetyl)-4-methylpyrazole bpym bipyrimidine CENTC center for enabling new technologies through catalysis CMD cyclometallation deprotonation COE cyclooctene DCM dichloromethane DFT density functional theory DMEP dimethyl((pyridinylmethylene)amino)ethylamine DMPP dimethyl((pyridinylmethylene)amino)propylamine DMSO dimethylsulfoxide dppbz Bis(diphenylphosphino)benzene dppe 1,2 bis(diphenylphosphino)ethane ESI-MS electrospray ionization mass spectroscopy Et ethyl fac facial ft3 cubic feet HNCNH 5-tert-butyl-1,3-bis(pyrazol-3-yl)benzene HNNNH 5-tert-butyl-1,3-bis(pyrazol-3-yl)pyridine HOMO highest occupied molecular orbital Hz hertz Inc incorporated IS internal standard kcal/mol kilocalorie per mole KIE kinetic isotope effect

LUMO lowest unoccupied molecular orbital m3 cubic meters Me methyl MeOH methanol MLC metal ligand cooperation NMR nuclear magnetic resonance NNH 5-(2-pyridyl)-3-tert-butylpyrazolate NNMe 5-(6-methyl-2-pyridyl)-3-tert-butylpyrazolate NNNEt 2-(5-tert-butylpyrazol-3-yl)-6 (diethylaminomethyl)pyridine Pd palladium Ph phenyl PP pyridinophane ppm parts per million Pt platinum R-H hydrocarbon SQD signal quality detector TACN 1,4,7-triazonane tBu tert-butyl THF tetrahydrofuran TON turnover number Tp hydridotrispyrazolylborate TUV tunable ultra-violet UPLC ultra performance liquid chromatography

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Compound Numbering Scheme

Et Pt(*NN)2 (B1) Pt(*NNN) CH3 (B4a)

Et Pt(*NNN) C6H5 (B4b)

Pt(*NN)(S(CH3)2)(CH3) (B2a or B2b)

Pt(*NNN)EtCl (B5)

Me Pt(*NN) (PPh3)(CH3) (B3)

Synthesis of Pt(HNNN)EtCl (B5a)

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Et 1 Et Pt(*NNN) Cl (B5b) Pt(κ -*NNN) (P(CH3)3)2CH3 (B8b)

H Me [Pt( NN) (S(CH3)2)(CH3)][BF4] (B6a/B6b) [Pt(HNNNH)Cl][Cl] (C1)

H Et Synthesis of [Pt( NNN) CH3][BF4] (B7). H H [Pt( NNN )Cl][BF4] (C2)

1 Et . Pt(κ -*NNN) (P(C5H6)3)2CH3 (B8a)

H H [Pt( NNN )NCCH3][(BF4)2] (C3)

xviii

H H H tBu Pt( NCN ) Cl (C4) [Li(THF)2]2[Pt(*NCN*) Cl] (C5A)

[Li2Cl(THF)4][Pt(*NNN*)CH3] (C6) tBu [Li(THF)2]2[Pt(*NCN*) Cl] (C4A)

Pt(HNCNH)tBuCl (C5) [Li(THF)]2[Pt(*NNN)CH2CN]2 (C7)

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H [PPN][Pt(*NNN*)CH3] (C8) [Li(THF)2]2[Pt(*NCN*) (CH3)] (C11)

H H [Pt( NNN )CH3][Cl] (C9) tBu [Li(THF)2]2[Pt(*NCN*) (CH3)] (C12)

H H 24 [Pt( NNN )CH3][BArF ] (C10)

tBu [Li]2[Pt(*NCN*) (C6H5)] (C13)

CH3 Pt( NNN*)(CH3)2(I) (C15)

H [Pt( N(P(C6H5)2)2)(CH3)(pyr-d5)][BF4] (D5)

H Pt( N(P(C6H5)2)2)(CH3)2 (D1a)

Pt2(µ-*N(P(C6H5)2)2)(µ-CH2)(pyr)2 (D6)

H t Pt( N(P( Bu)2)2)(CH3)2 (D1b)

H [Pt( N(P(C6H5)2)2)2][BF4]2 (D2)

Pt(*N(P(C6H5)2)2)2 (D3)

H t Pt( N(P( Bu)2)2)(Cl)2 (D4)

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Acknowledgements

I would like to begin by thanking everyone who has helped me along my journey to complete my Ph.D. I have wanted this ever since I was a little kid and I am forever grateful for those who have contributed to helping make this possible. To my 8th grade chemistry teacher Mr. Schreck at Queens Grant Community School, thank you for being the first teacher who established my love for chemistry. I hope to one day see you again and thank you in person for setting me on my path. To my parents, Ingrid and Andy; my brother, Connor; and to the rest of my family I discussed my experiences with along the way, I love you all. Dad, you have inspired me ever since I was little. I have always looked up to you and have always wanted to be a Ph.D., just like you. Glad I could finally join the club! Mom, thank you for all your words of wisdom. When I was down and struggling, talking with you lifted me up and you gave me the strength to continue. You always set my mind right and encouraged me to see the brighter side. Connor, you have also always been there for me and you are so fun to hang around. You have become a fine man and I am so proud to call you my brother. To my grandma and nana who recently passed away. I love you both so much - you’re often on my mind and I miss you both more than I can put into words. To the rest of my family: Karen and Mike, Rob and Janet, Ken, Bonnie and Mark and co., Nils and Consuelo and Jim and Karen; you all are so amazing and thank you all for providing a strong support for me to lean on. Thank you, Karen Goldberg, for being a terrific mentor throughout my graduate career. The love and passion you show for solving complex chemistry problems first drew me to your lab and I was honored when I was given the opportunity to join. While moving the lab across the country had its challenging moments, I was happy to do it with you and the wonderful people in your lab. All in all, with your guidance, I grew into the scientist that I am today, and I am forever grateful. I learned so much from you; you are beyond an exceptional chemist and I wish you all the success in the world at UPenn. To all my fellow scientists that I met along the way, thank you for making graduate school fun and providing an outlet from science. I met so many amazing people in both Seattle and in Philadelphia and while I can’t mention everyone, you all mean so much to me. To the past and present Goldberg lab: Wilson, Cameron, Marie, Tyler, Zu, Karena, Irene, Jon, Ash, Tim, Magnus, Amy, Kelly, Hannah, Byongjoo, Sophie, Alex, Drew, Evan, Sabrine, Anant and Walter, thank you for your help not only in the lab but also your friendship – I’ll never forget you all. To the now Dr. Michael Enright, you have been an amazing friend and I miss all the fun we had going on hikes, touring the city and just hanging out watching sports. I hope to see you soon and keep our friendship going for years to come. To Dr. Louise Guard and Dr. Jonathan Kuo, I cannot thank both of you enough for all that you did for me. You both are phenomenal chemists and friends and helped me succeed when I couldn’t. I’ll never forget you both.

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And finally, to Jenn Lee, I am so lucky to have met you and am continuously falling more in love with you every day. While the long-distance relationship was tough while I was at UPenn, thank you for the love and support you showed me while I was thousands of miles away. It was tough and I missed you so much, but we made it! Now the real journey begins, and I couldn’t think of a better person to share it with.

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Chapter Contributions

Much of the work detailed in the following chapters would not have been possible without the valuable efforts of NMR and X-ray facility staff at the University of Washington and the University of Pennsylvania. Chapter 2. Sarah E. Flowers, Maike Blakely and Werner Kaminsky for the crystallographic characterization of B4b and B5 Michael Gau and Pat Carroll (Upenn) for the crystallographic characterization of B4a Chapter 3 Sarah E. Flowers, Maike Blakely and Werner Kaminsky for the crystallographic characterization of C5a Michael Gau and Pat Carroll (Upenn) for the crystallographic characterization of C2, C3, C4, H H (Pt4(*NCN*)( NCN )3), C6, C8, C9, and C11 Chapter 4 Sarah E. Flowers, Maike Blakely and Werner Kaminsky for the crystallographic characterization t of Pt(*N(P( Bu)2)2)2 Michael Gau and Pat Carroll (Upenn) for the crystallographic characterization of D3 and D6.

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Chapter 1

Introduction

1.1 A Need to Reduce Hydrocarbon Flaring

Hydrocarbons are the main component of petroleum and natural gas. While petroleum is made up of a large array of saturated and unsaturated hydrocarbons, natural gas is primarily comprised of low molecular weight alkanes, mainly methane. These valuable resources supply over 90% of the carbon feedstocks to the chemical industry.1 In fact, total world consumption of natural gas in 2018 was over 10 billion m3/day and total world consumption of petroleum in 2018 was about 100 million barrels/day.2,3 The Bakken formation in North Dakota is one of the largest contiguous deposits of oil and natural gas in the United States and contains a predicted 7.4 billion barrels and 6.7 trillion ft3, respectively, of proven resources.4 Respective crude oil and natural gas production for the Bakken region has increased to a record 1.5 million barrels/day and 2.2 million ft3/day in October 2019.5 Petroleum and natural gas are ever-present in the chemical industry economy and it is crucial to efficiently utilize these resources.

Petroleum and natural gas (or “associated gas”) from oil fields such as the Bakken

Formation are often extracted through a process known as hydraulic fracturing (or “fracking”).

Here, a specially engineered fluid is injected down a well, which penetrates shale and forces the less dense oil and gas upwards towards the well site.6 A lack of adequate natural gas storage and/or transportation infrastructure from the fracking well sites result in significant “flaring” or the controlled burning of associated gas to mitigate more serious environmental impacts.7 In fact, oil extraction in March of 2019 through the Bakken region produced 560 million ft3 of flared gases resulting in a wasted $6.7 million if extrapolated throughout the year (based on reported US price in March 2019 of $4.33/1000 ft3).8,9 Flaring occurs on such a large scale in North Dakota that it can even be seen from space (Figure 1.01).10 Even though the North Dakota oil and natural gas extraction industry has spent billions of dollars on infrastructure already, gas liquification, storage

Figure 1.0 1 Image of light pollution across the central United States. Image was produced by NASA using data acquired in April and October 2012. Image downloaded from https://geology.com/articles/oil-fields-from-space/ in November 2019.

2 and transportation is expensive; there is little economic incentive to completely prevent flaring and keep up with the increasing gas production in the state.7,11

Current industrial methods of hydrocarbon functionalization require large amounts of energy. There is no industrially viable method for directly and selectively oxidizing methane, which could help provide an economic incentive. Currently, hydrocarbons are initially converted to syn- gas, a mixture of H2 and CO. This process requires high temperatures (ca. > 800 °C) and often results

12,13 in over-functionalization to CO2. A separate reaction (ca. > 170 °C) then transforms syn-gas to functionalized products, such as methanol.14 In addition, hydrocarbon functionalization plants (or

“gas-to-liquid” plants) require large initial costs. In 2018, ONEOK, Inc. company began the production of the Demicks Lake I and II natural gas processing plants (and associated infrastructure) in the Bakken region, which are expected to cost a total of $1.7 billion and add about 1.1 billion ft3/day plant capacity.15 While the production of plants like these help the flaring situation, natural gas production in the North Dakota Bakken region exceeded 2.4 billion ft3/day in July 2018, and increased by about 8.7 % from July 2018 - December 2018.15,16 New methods need to be developed to avoid this costly, atom inefficient and energy intensive procedure.

A highly desirable process would be the direct transformation of hydrocarbons like methane into valuable commodity chemicals in a single step. This is considered to be one of the “holy grail” reactions of catalysis and numerous organic functionalized compounds have been targeted such as methanol, formaldehyde, ethylene, benzene and others.17,18 Perhaps most intriguing in this list is methanol, as it could be formed by methane partial oxidation from O2 in the atmosphere.

Heterogeneous catalysts offer practical aspects for use in industrial chemistry, such as ease of product and catalyst separation, and additionally have demonstrated stability at high temperatures and pressures. Recently reported heterogeneous catalysts have exhibited low conversion of methane

(~15 %) and good selectivity (> 60 %) for methanol (over such products as HCOOH, CH3COOH

19,20 and CO2), and are still not efficient enough for industrial applications. As reaction pathways can be difficult to study in heterogeneous systems, therefore often precluding rational catalyst design, we turn to homogeneous catalysis to improve selectivity and explore other potential routes for alkane partial oxidation.

1.2 Homogeneous Partial Oxidation of Methane, an Overview

Difficulties in methane functionalization originate from the inert nature of alkanes: high C-

H bond strengths (c.a. 105 kcal/mol for methane), pKas (c.a. 50 in DMSO for methane), and large orbital energy mismatches (no low energy empty orbitals or no high energy filled orbitals) all contribute.21 Additionally, the functionalized products formed are often more reactive than the

22 starting hydrocarbons (i.e. methanol C-H bond strength of 96 kcal and CH3O-H pKa of 29 in

DMSO.23 Organometallic complexes allows us to address these challenges and successful homogeneous methane partial oxidation has been observed.13,24

Noble metal complexes have shown a great propensity to activate strong bonds, the first step in functionalization.25 In the late 1970s, Shilov and coworkers were the first to describe a platinum complex which could catalytically functionalize methane to methanol and chloromethane under aqueous conditions, albeit with low conversion rates (Figure 1.02).13 Here, methane C-H activation

II II is achieved by a Pt Cl2(H2O)2 catalyst to form HCl and a Pt -CH3 complex. While there has been extensive experimental and computational evidence for R-H oxidative addition at PtII model complexes to generate PtII-R, the alternative electrophilic C-H activation for the Shilov system

26 II cannot be ruled out as operational in the Shilov catalyst system. After formation of the Pt -CH3

IV II IV complex, an equivalent of added Pt oxidizes the Pt -CH3 to a Pt -CH3 complex. Nucleophilic

Figure 1.0 2 Catalytic system developed by the Shilov transforming methane to methanol (and chloromethane). attack by water then generates methanol and regenerates the PtII catalyst. Through extensive isotopic labeling experiments, Bercaw and Labinger conclusively showed that PtIV acted as an external oxidant, indicating potential for use of other oxidants.27 Additionally, conversion rates of methane to methanol were limited due to the poor thermal stability of PtII catalyst, which decomposed to metallic Pt at high temperatures. Heterogeneous metallic Pt is known to be an active but unselective catalyst for hydrocarbon oxidation.28 Overall, even though this system was not practical, it demonstrated the potential for electrophilic methane activation by PtII and highlighted the need for the discovery of more practical oxidants.

To improve upon Shilov’s seminal work, oxidants other than PtIV have been explored for

II 2- IV alkane oxidation catalysis with Pt catalysts. While substituting Cl2 or S2O8 for Pt in the Shilov system gave only a few turnovers, it did confirm that methane partial oxidation is not dependent on the presence of PtIV.29,30 Furthermore, functionalization of ethanesulfonate (a hydrophilic alkane replacement) to 2-hydroxyethanesulfonic acid with stoichiometric amounts of CuCl2 was shown

2- 31 using a PtCl4 catalyst and up to 43 turnovers were demonstrated. A competition experiment in

IV II 2- water between Pt and CuCl2 revealed the latter increases the rate of oxidation of [Pt Cl3(CH3)] to

IV 2- 32 [Pt Cl5(CH3)] by an order of magnitude (ca. koxidation/kprotonation = 191 vs 20). Unfortunately, even for these alternative oxidants, eventual formation of metallic Pt was observed, just as it was in the original Shilov system.

A significant advance in the electrophilic functionalization of methane was reported in 1993 by Periana and coworkers. HgII was found to catalyze methane oxidation to methylbisulfate in concentrated sulfuric acid, proceeding with 85 % selectivity and 50 % conversion.33 An improvement on this system, based on, the Pt based Catalytica process, was also reported by the

Periana group.34 Like the HgII system, the Catalytica process converts methane to methyl bisulfate.

In the Catalytica system, a bipyrimidine (bpym) ligated PtII precatalyst in concentrated sulfuric acid is employed (Scheme 1.01). An even higher conversion (> 70 %) of methane and similar selectivity

Scheme 1.01. Catalytica reaction

for methyl bisulfate (> 80 %) was achieved, resulting in an overall yield of 72 %. The methyl bisulfate product can then be extracted onto methanol by hydrolysis. A similar mechanism to the

Shilov system was proposed, where a primary difference is oxidation by sulfuric acid solvent, instead of PtIV. Additionally, while little experimental evidence has been disclosed, computational work

35 supports electrophilic C-H activation proceeding through a Pt-(σ-CH4) complex. In all, the

Catalytica system is the most efficient homogeneous methane functionalization effort to date.

However, several problems have plagued it from being commercially relevant, such as high product separation cost and insufficient reaction rates. In addition, efficient methane functionalization only occurs in concentrated sulfuric acid. As water is generated during the reaction, there is a supplementary cost of re-concentrating sulfuric acid solvent.36

While previous systems have achieved the successful functionalization of methane, a practical method with an economically viable oxidant has yet to be discovered. For large scale applications, the most cost-effective choice is oxidation by O2. O2 is an inexpensive, abundant and environmentally benign oxidant. While there has been some limited success of methane oxidation

II II III with a Pt catalyst using either Cu /O2 or Fe /O2 combinations as oxidants, the catalyst deactivated

36,37 after around 50 turnovers of methanol production. Stoichiometric O2 oxidation of a

II bis(dimethylpyrazolyl)acetate (NNO) or dipyridyl methanesulfonate (dpms) ligated Pt -CH3

IV IV complex to a Pt (NNO)(CH3)(OH)2 or Pt (dpms)(CH3)(OH)2 complex, respectively, was

IV demonstrated in H2O solvent, with further release of methanol from Pt at elevated temperatures in the latter example.38,39 However, C-H activation of the resulting product to close a potential catalytic cycle was never demonstrated. A system which utilizes O2 as a selective oxidant in methane functionalization remains a “holy grail”.

1.3 Designing A New System for Alkane Functionalization

The need for a new route to accomplish catalytic methane functionalization is apparent from analysis of the several known systems (Section 1.2). While the most efficient system to date,

Catalytica, built upon Shilov’s seminal work, choice of solvent/oxidant and low reaction rates precluded practical use. Partial oxidation by abundant, benign and inexpensive O2 is desired and a productive pathway as described in Figure 1.03 can be envisioned. Here, a low valent Mn complex

Figure 1.0 3 Proposed catalytic cycle for methane functionalization using O2. is proposed to activate an R-H substrate through oxidative addition, a step for which there is significant precedence.24,25 The oxidized Mn+2 complex undergoes oxygen insertion into the Mn+2-

n+2 n+2 n+2 II R or M -H bond to form a M -(OOR) or M -(OOH) complex. Oxygen insertion to Pt -CH3

II 40,41 II complexes have been described and form Pt -OOCH3 complexes. In addition to Pt , several

IV IV examples of O2 insertion into the Pt-H bond of higher valent Pt -R2H complexes to form Pt -

42,43 IV IV (OOH)(R)2 have previously been described. Decomposition of Pt -(OOH)(R)2 to form Pt -

42,43 n+2 (OH)(R)2 has also been reported. The M oxygen insertion product would then reductively eliminate R-OH to reform the low valent Mn complex. Evidence of C-O reductive elimination from

PtIV has previously been described; thermolysis of a 1,2-Bis(diphenylphosphino)ethane (dppe) or

IV 1,2-Bis(diphenylphosphino)benzene (dppbz) ligated Pt (CH3)3(OR) (R = Aryl, Acetate)

II 44,45 complexes form CH3OR and Pt (dppe/dppbz)(CH3)2. While there is precedence for all of the individual steps in this possible catalytic cycle, a full catalytic cycle has yet to be realized in practice.

The envisioned catalytic cycle (Figure 1.03) has potential, yet one major problem exists: the low valent metal complex. Low valent metals are often reactive towards other bonds present in the reaction mixture, apart from the desired C-H bond, such as the alcohol product, O2, and N2.

To start, not only are functionalized C-H bonds weaker than unfunctionalized C-H bonds, but other reactive bonds are present.34 O-H oxidative addition to low valent complexes have been postulated

46,47 numerous times and direct observation has even been documented. Oxidative addition of O2 to low valent metals is also known and has been studied, leading to another unproductive side reaction for the targeted catalytic cycle.48,49 If our goal is to develop aerobic oxidations of alkanes, we need to find a system that would be more tolerant of O2, water, and the potential functionalized products. PtII complexes are an attractive target; they are often stable in solution and they are already known to functionalize methane C-H bonds at elevated temperatures (Section 1.2). While

R-H is known to undergo oxidative addition reactions at PtII, does a pathway exist which might activate the C-H bond without undergoing a net oxidation state change at the metal?

To draw inspiration, we can turn to a CH4 analog, H-H. Dihydrogen is a nonpolar small molecule and contains a similarly strong bond (ca. H-H BDE = 104 kcal/mol).50 Additionally, it is

n known to initially react with transition metals to form dihydrogen complexes (M -(H2)) through a σ

51,52 n+2 interaction with the metal. Once coordinated, homolytic cleavage to form a M -(H)2 complex can occur through an oxidative addition route. However, heterolytic cleavage can also occur

n 51 intermolecularly or intramolecularly, as M -(H2) complexes are generally acidic. The pKa of Re(η-

2 53 C5Me5)(CO)(NO)(η -H2) was estimated to be around -2 in CH2Cl2, as it protonated diethyl ether.

Heterolytic cleavage by deprotonation of a dihydrogen complex is intriguing, as it circumvents formal oxidation at the metal. Furthermore, intramolecular dihydrogen cleavage at a benzoquinolate

III III 54 ligated Ir center was observed; the amine deprotonated the Ir -(H2) moiety (Scheme 1.02a). This example is powerful, as it demonstrates the power of the ligand to assist in deprotonating strong

II bonds. Intramolecular heterolytic H2 activation by aminophosphine ligated Pt is also known

(Scheme 1.02b).55

III II Scheme 1.02. Heterolytic cleavage of H2 by an a) Ir complex and b) Pt complex

Nonpolar R-H bonds can also initially interact with metal centers like dihydrogen, forming a very reactive σ-alkane metal complex. The formation of σ-complex intermediates has been postulated from H-D scrambling experiments and inverse kinetic isotope effects in the reduction elimination reaction of higher valent metal alkyl hydrides.25 The σ-alkane metal complex,

I + 56 [Rh (PONOP)(CH4)] , was even observed and spectroscopically characterized at -110 °C.

Observance of this σ-methane complex, the simplest alkane analog to M-(H2) complexes, provides further insight into initial reactivity of R-H with transition metals. Furthermore, hydrocarbon substrates could potentially react with transition metals in an analogous way to H2. Here, the C-H bond of the σ-alkane complex could be deprotonated, either intermolecularly or intramolecularly

(Figure 1.04). No net change in oxidation state in the metal center would additionally occur.

Figure 1.0 4 Activation of R-H substrate by an intermolecular (top) or intramolecular (bottom) pathway.

Intermolecular C-H activation with transition metals through an oxidative addition route has been extensively studied,57–59 yet deprotonation of the sigma complex by an external base has yet to be directly observed (Figure 1.04, top pathway). However, electrophilic aromatic substitution of arene C-H moieties by transition metals is a similar process. Arene C-H substrates are activated by electrophilic metals, like PtII, and are postulated to initially form a metal substituted Wheland intermediate (Figure 1.05a).60 The reactive metal-based Wheland intermediate has even been isolated and characterized at a PdII complex.61 The intermediate is

11 subsequently deprotonated to rearomatize the arene. Such C-H activations have been postulated in the metalation of multidentate ligands.62,63

Intramolecular C-H activation has been observed as well, where noble metals are known to participate in concerted metalation deprotonation (CMD) reactions.64,65 CMD, demonstrated in

Figure 1.0 5 Activation of R-H substrate by an (a) electrophilic aromatic substitution pathway, and (b) concerted metalation deprotonation pathway.

Figure 1.05b, is a powerful type of C-H activation, where the metal and a basic ligand (typically a carboxylate) heterolytically cleave an R-H bond in a concerted manner without a formal metal oxidation state change.66 The protonated weakly binding ligand then dissociates from the metal complex. If a basic site could be incorporated into the ligand backbone, then the protonated ligand could later transfer the acidic proton back to the substrate, either inter or intramolecularly.

Heterolytic activation of C-H substrates by a CMD route is attractive, yet dissociation of acidic ligand after C-H activation could hinder its utilization in future M-C functionalization reactions. Loss of ligand could be addressed by utilization of a ligand basic site, which would remain attached after C-H activation. Notably, the Milstein67,68 and Huang69 group have observed C-H activation of benzene substrate (Figure 1.06) by a relevant proposed pathway. In the Milstein group

I I Figure 1.0 6 Activation of C6H6 substrate by (a) Ir /Rh (PNP) complexes, and by a (b) RhI(PNNNP) complex. example, a bis-phosphinoethylpyridine (PNP) ligated RhI and IrI complexes activated benzene C-H substrate after ligand methylene CH2 deprotonation (Figure 1.06a). Reduced temperatures (-78 °C) experiments indicate initial reactivity at the metal to form the IrIII alkyl hydride, followed by intramolecular ligand protonation of the Ir-H at the deprotonated methylene site by 1H NMR spectroscopy.67 The ability of the ligand to aid in deprotonation was furthered by Huang, where a

13 basic N in a RhI species was proposed to aid in activation of the benzene C-H bond in a similar manner to that proposed by the Milstein group (Figure 1.06b).69 C-H activation reactions by the

Milstein and Huang group are proposed to be promoted by a dearomatization/aromatization reorganization within the system.

While previous examples have demonstrated that C-H bond activation by a ligand aided intramolecular route is possible with IrI/RhI complexes, similar transformations using PtII complexes have not been extensively explored. The premise of the following work would be to construct a PtII complex with a ligand based basic site which could be used to either deprotonate the PtII-(σ-(R-H)) moiety or the resulting PtIV-(R)(H). If such a C-H activation system is to be developed, it is reasonable to predict that a species which could activate a C-H bond might be quite reactive. To address this, we sought to examine the microscopic reverse reaction (or formation of C-H bonds) to gain an understanding into the forward reaction (C-H activation). Extensive studies have been performed on the protonation of PtII-R compounds (initially forming PtIV-(R)(H) species) and often eliminating R-H and the appropriate PtII complex through reductive elimination.25 If a basic N site, is incorporated into the ligand, one can ask will similar reactivity be observed exclusively at the metal, or will the proton of the C-H substrate migrate to the ligand?

1.4 Dissertation Summary

The following chapters describe the syntheses, characterization, and reactivity studies of PtII- alkyl complexes with reactive N sites on the ligand, as shown in the bottom intramolecular pathway in Figure 1.04. The focus of this work is geared towards a greater understanding of C-H activation reactions by examining the microscopic reverse, or by what conditions PtII-alkyl species with an N-

H proton in the ligand generate alkane (C-H formation). The findings from these studies expand the

14 scope of reactivity for PtII-alkyl systems which contain a ligand basic site. Specifically, pyrazolate

II II (Chapter 2 and 3) ligated Pt -CH3 and aminobisphosphine (Chapter 4) ligated Pt -(CH3)2 complexes were investigated.

Chapter 2 describes the synthesis and characterization of bidentate 5-(6-methyl-2-pyridyl)-

3-tert-butylpyrazolate (NNMe) and tridentate 2-(5-tert-butylpyrazol-3-yl)-6

Et II II (diethylaminomethyl)pyridine (NNN ) ligated Pt -Cl and Pt -R (R = CH3 and C6H5) complexes.

Me Et II Once formed, protonation and thermolysis studies of (NN ) and (NNN ) ligated Pt -CH3 complexes were performed to investigate parameters for alkane formation (microscopic reverse of

C-H activation). Chapter 3 additionally examines the synthesis and characterization of 5-tert-butyl-

1,3-bis(pyrazol-3-yl)pyridine (NNN) and 5-tert-butyl-1,3-bis(pyrazol-3-yl)benzene (NCN) ligated

II II II Pt -Cl and Pt -CH3 complexes. The reactivity of Pt -CH3 complexes was evaluated with Brønsted acids and the reactivity with electrophilic methyl iodide was additionally explored. Chapter 4 discusses synthesis and characterization of both known and novel bis(phosphino)amine ligated PtII complexes, taking advantage of steric congestion to promote productive reactivity. Successful methane elimination (C-H formation) was demonstrated from a ligand protonated bis(phosphino)amine Pt(CH3)2 species, however, the resulting paramagnetic complex could not be characterized and thus requires further investigation.

1.5 Notes for Chapter 1

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Chapter 2

Synthesis and Reactivity of Bidentate and Hemilabile Pyrazolate Ligated Pt Complexes

2.1 Introduction

Natural gas, a significant feedstock for the chemical industry, is comprised of methane and

light alkanes.1 Viable routes that do not involve high energy steam reforming for the conversion

of methane to higher value-added products are not yet available and the development of such

2 II systems remains a challenge. Aqueous electrophilic functionalization of methane (CH4) by a Pt -

Cl complex to generate methanol was first demonstrated by Shilov almost 50 years ago.1 While

remarkable, this system was deemed economically impracticable, as it required stoichiometric

amounts of a PtIV oxidant. Since then, many studies of both stoichiometric and catalytic C-H

functionalization using PtII complexes have been carried out as the community seeks to identify

more viable catalyst systems that could potentially be used on commercial scale for alkane

functionalization. 2,3

Activation of C-H bonds by PtII, the first step in C-H functionalization, often proceeds by

oxidative addition, generating a PtIV-hydrido alkyl complex. The oxidized PtIV hydrido alkyls are

typically less thermodynamically stable relative to their PtII counterparts and methane. Thus,

significant insight into this difficult transformation has been gained through the study of the more

Portions of this chapter have been adapted from: Zahora, B. A.; Gau, M. R.; Goldberg, K. I. In Preparation 22 accessible microscopic reverse reaction: reductive elimination of alkane from PtIV hydrido alkyl complexes. These C-H coupling reactions can be carried out by either using isolated octahedral PtIV hydrido alkyl complexes or by protonation of square-planar PtII alkyl compounds. In the latter case, upon addition of a Brønsted acid to a square planar PtII-alkyl compound, either a PtIV alkyl hydride species is spectroscopically detected and subsequently undergoes C-H reductive elimination, or the

C-H coupled alkane product with a PtII species is observed directly, without evidence of an intermediate. If no intermediate is observed, it is challenging to conclusively determine whether the reaction takes place by direct protonation of the PtII-alkyl bond or by protonation of the metal to produce a PtIV-alkyl hydride intermediate. Most data support that the initial site of protonation in PtII species is the PtII center with formation of a PtIV alkyl hydride species.4–7 However, computational

Figure 2.0 1 Ligand examples employed which have allowed isolation of PtIV methylhydrides work by Bercaw and Lin has suggested that with electron deficient ancillary ligands, direct

II IV 8 protonation of the Pt -CH3 bond, bypassing Pt , may occur.

It is also significant that most studies of reductive elimination of alkanes from octahedral PtIV alkyl hydride complexes support that the concerted C-H bond forming step occurs from a five- coordinate intermediate.9 This mechanistic understanding has made it possible to stabilize octahedral

PtIV alkyl hydride complexes by using strongly bound ligands that resist dissociation. In particular, facially coordinating tridentate nitrogen ligands have been used to stabilize PtIV dimethyl hydride

23 complexes (Figure 2.01).10–12,13 Thus, PtIV dimethyl hydride complexes of tris(pyrazolyl)borate (Tp),

1,4,7-triazacyclononane (TACN), and bis(pyridylmethyl)amine have all been reported (Figure 2.01).

II All these ligands coordinate to Pt (CH3)2 species in a bidentate fashion and upon protonation, the third arm coordinates, generating a fac chelating ligand arrangement in the stable octahedral PtIV product. Whether the metal is protonated directly or if the ligand is the kinetic site of protonation, followed by proton transfer to the metal is challenging to ascertain. Protonation at the ligand has been observed for pyridinophane (PP)14 and

15 II dimethyl((pyridinylmethylene)amino)(ethyl/propyl)amine (DMEP/DMPP) ligated Pt (CH3)2 species (Scheme 2.01).

Scheme 2.01 (a) Protonation of pyridinophane ligated PtII (b) Protonation of DMEP(n=1) /DMPP(n=2) ligated PtII

H For the PP system, a small amount (8 % yield) of the protonated ( PP)Pt(CH3)2 intermediate was observed by 1H NMR spectroscopy at -80 °C (Scheme 2.01a). Warming to -30

°C promoted conversion to the oxidized PtIV hydride product. Here, it was proposed that the pyridyl nitrogen helps shuttle the proton from the ligand to the metal center.14 In contrast, the

H H II ligand protonated ( DMEP/ DMPP) Pt (CH3)2 species were reported to be stable for several hours in solution at room temperature before methane elimination (Scheme 2.01b).15 It is possible that the N-H moiety of the protonated HDMEP/HDMPP ligand helps to shuttle the proton to the metal

25 in a similar fashion to the protonated HPP ligand described above. While in the vast majority of

DMEP/DMPP complexes, the ligand adopts a mer-orientation, there are several examples where a fac-orientation is exhibited.16,17 The fac-orientation would be more suited to aid in direct proton transfer to the open position on a d8 metal center. While no PtIV intermediates were observed prior to methane liberation for the DMEP/DMPP complexes, the addition of two equivalents of HCl to

II IV (DMEP)Pt (CH3)2 did form an observable Pt hydride at low temperatures, which quickly liberated methane upon warming to -30 °C. No PtIV hydride was observed when acid was added

II 15 to (DMPP)Pt (CH3)2.

Our interest lies in exploring ligands that can accept a proton, but do not have the ability

IV II to reorient and stabilize Pt hydride products. Our studies of the protonation of two Pt -CH3 complexes bearing either a bidentate or tridentate ligand and further C-H coupling from these

II compounds are described below. Previous examples of square planar bidentate ligated Pt (CH3)2 complexes with an available basic nitrogen on the ligand have contained either dangling N moieties or N moieties enclosed in aromatic based systems.14,15,18 Protonation will then either take place at

II the metal or at the ligand. In bidentate systems with dangling N moieties, such as in LPt (CH3)2, two arms of a potentially tridentate ligand L are coordinated in the PtII complex. The third arm bears a basic nitrogen that can either be protonated or can bind to the metal when the metal center is protonated to yield an octahedral PtIV hydride complex. When a PtIV dimethyl hydride species is formed, the stabilized product adopts a fac orientation.

II We sought to prepare bidentate and tridentate Pt -CH3 complexes wherein the available basic site for protonation is not available to bind to the metal. The basic site is physically constrained to only act as a base and not a ligand. To achieve this arrangement, bidentate and tridentate pyrazolate- supported ligands, 5-(2-pyridyl)-3-tert-butylpyrazolate (HNNR) (R = H or Me) and 2-(5-tert-

26 butylpyrazol-3-yl)-6-(diethylaminomethyl)pyridine (HNNN)Et, which have been complexed to several metals (e.g. RuII, IrIII, CoII and FeII),19–21 were used. Notably, these ligands have not yet been

8 H R H Et found in square planar d complexes. Herein, we describe metalation of ( NN ) and ( NNN) to

8 II II afford d square planar Pt complexes. These pyrazolate ligated Pt -CH3 species will have basic nitrogen sites on the ligands available for protonation.

2.2 Results and Discussion

2.21 Synthesis of Bidentate Ligand Supported Pyrazolate PtII-Complexes

Figure 2.0 2 (a) Metalation of HNNH (b) Metalation of HNNMe

The mono pyrazole ligands, 5-(2-pyridyl)-3-tert-butylpyrazolate (HNNH) and (5-(6-methyl-

2-pyridyl)-3-tert-butylpyrazolate (HNNMe) were prepared in high yield according to literature procedures.22 A notable difference in these two ligand types is the methyl substituent in the 6- position, which was initially installed to prevent potential rollover activation of the α-pyridyl C-H bonds. This reactivity has been observed at high temperatures and can result in undesirable Pt cyclometalated complexes from C-H activation of ligand.23 A 1:1 molar ratios of ligand to metal

H H H Me H H coordinated differently for NN and NN with [Pt(µ-S(CH3)2)(CH3)2]2: two NN ligands coordinate to PtII while only one HNNMe ligand coordinates to PtII (Figure 2.02). Addition of HNNH

28 to the Pt-CH3 starting material at room temperature yielded methane, S(CH3)2, and a major and multiple minor Pt-containing species by 1H NMR spectroscopy. Further heating of the reaction mixture to 100 °C converged the multiple minor Pt species to the one major complex, which did not

1 display a Pt-CH3 resonance, or a Pt-S(CH3)2 resonance by H NMR spectroscopy. In addition, a Pt-

H resonance was not observed upfield in the 1H NMR spectrum. The same major product was also observed by 1H NMR spectroscopy (without the observance of any free ligand) when 2 equivalents

H H 1 of NN was added to [Pt(µ-S(CH3)2)(CH3)2]2 and subsequently heated. By H NMR spectroscopy, a single species was observed without a Pt-CH3 or Pt-S(CH3)2 resonance. The main species is

H proposed to be the homoleptic Pt(*NN )2 complex (B1) [Note: * refers to deprotonated pyrazolate

N]. Formation of homoleptic *NNH ligated PdII complexes have been previously described in metalation reactions due to the lack of sterics in the 6 position of the ligand pyridyl moiety.24–26 1H

H NMR assignments of reported Pd(*NN )2 is in agreement with B1.

H Me Metalation of NN at 60 °C with [Pt(µ-S(CH3)2)(CH3)2]2 resulted in

Me 1 Me Pt(*NN )(S(CH3)2)CH3 (Figure 2.02a). The H NMR spectrum of Pt(*NN )(S(CH3)2)CH3

2 exhibits a methyl resonance at 1.39 ppm with JPt-H coupling of 84 Hz, and a S(CH3)2 resonance at

3 2.49 ppm with a JPt-H of 52 Hz. There are two potential conformers for this product (B2a/B2b,

Figure 2.02). The Chen group have previously reported bidentate 3,5-bis(2,6-

II dimethylphenyliminoacetyl)-4-methylpyrazole (bdmimp) ligated Pt -(CH3)(S(CH3)2) complexes.

2 Here, when Pt-CH3 is trans to a pyrazolate moiety, JPt-H is smaller (69 Hz) than when trans to a

2 27 pyridyl moiety (79 Hz). This suggests B2a over B2b from examination of the JPt-H value (80.3 Hz).

Complex B2a/b was found to be unstable to silica and alumina (neutral and basic) and attempts to purify and fully characterize B2a/b were unsuccessful. The formation of B2a/b instead of a homoleptic complex can rationalized from the sterics of the methyl substituent in the 6 position of

29 the bidentate HNNMe ligand. To isolate a more stable complex, the reaction of B2a/b with a more

1 31 1 strongly bound ligand than S(CH3)2, namely PPh3, afforded a single species by H and P{ H} NMR

1 spectroscopy. Liberation of S(CH3)2 was observed in the H NMR spectrum and the appearance of a

31 1 new P shift at 18.22 ppm with corresponding Pt satellites ( J(Pt-P) = 4186 Hz) allowed for the

Me 1 assignment of Pt(*NN )(CH3)(PPh3) (B3). By H NMR spectroscopy, B3 exhibited a Pt-CH3

2 3 resonance at 0.97 ppm ( JPt-H = 80 Hz, J(H-P) = 5.1 Hz). B3 was also unstable to isolation; the change

2 from B2a/b to B3 does not appear to have a significant effect on the JPt-H of the Pt-CH3 or on ease of isolation.

2.22 Preparation of (NNN)Et Ligated PtII Complexes

Figure 2.03. Metalation of HNNNEt to PtII to form B4a, B4b and B5. Thermal Ellipsoid plot Et Et Et of Pt(*NNN) CH3 (B4a) and Pt(*NNN) Cl (B4b) and Pt(*NNN) C6H5 (B5) at 50% probability (CH2Cl2 in B4a omitted for clarity and H-atoms omitted for clarity). Selected bond distances for B4a (Å): N(1)-C(6) 1.387(6) Å, N(2)-C(3) 1.355(8) Å, Pt-N(13) 1.948(8) Å, Pt-C(22) 2.062(6) Å. Selected bond distances for B4b (Å): N(2)-C(6) 1.377(6) Å, N(3)-C(8) 1.349(8) Å, Pt-N(1) 2.015(4) Å. Selected bond distances for B5: N(3)-C(7) 1.38(1) Å, N(4)-C(9) 1.35(1) Å, Pt-C(18) 2.050(7) Å, Pt-N(1) 1.996(7) Å.

Addition of 1 equiv. 2-(5-tert-butylpyrazol-3-yl)-6 (diethylaminomethyl)pyridine (HNNN)Et

Et to [Pt(µ-S(CH3)2)(CH3)2]2, Pt(S(CH3)2)(C6H5)2 or Pt(S(CH3)2Cl2 affords Pt(*NNN) CH3 (B4a),

Et Et Pt(*NNN) C6H5, (B4b) and Pt(*NNN) Cl (B5) in 79 %, 81 %, and 67 % isolated yield, respectively

1 13 1 (Figure 2.03). An excess of NEt3 is additionally required to form B5. The H and C{ H} NMR

2 II spectra are consistent with assignment of B4a with a JPt-H = 78 Hz observed for Pt -CH3 group. The

1H and 13C{1H} NMR spectrum is consistent with assignment of B4b with three additional aryl

1 resonances observed in the H NMR spectrum for the Pt-C6H5 ligand. Diastereotopic methylene proton signals are observed in the 1H NMR spectra of B4a, B4b, and B5, consistent with chelation of the diethyl amine moiety. B4a and B4b were additionally characterized by elemental analysis, while complex B5 was characterized by high resolution mass spectroscopy (m/z = 557.2187). The monomeric nature of B4a, B4b, and B5 were also crystallographically confirmed (Figure 2.03). C-

N bond distances within the pyrazolate group for B4a, B4b, and B5 suggest that the C-N bond associated with the non-coordinated N has slightly more double bond character than that of the coordinated one. In B1, these values are 1.355(8) Å (N(2)-C(3)) versus 1.387(6) Å (N(1)-C(5)).

Similar bond length differences are seen in the crystal structures of other pyrazolate ligands bound to metals.20,21 Together, the absence of an N-H resonance in the 1H NMR spectrum and the lack of counter anions in the solid-state structure, suggests that the pyrazolate moiety is deprotonated in B4a,

B4b, and B5. Additionally, the change in the Pt-pyridine bond distance (1.948(8) Å (B4a) vs

1.996(7) Å (B4b) vs 2.015(4) Å (B5) suggests a trans-influence trend where CH3 > C6H5 > Cl.

2.23 The Reactivity of B2a/B2b and 4a with Acid

II With bidentate (B2a/B2b) and hemilabile tridentate (B4a) Pt -CH3 complexes in hand, their reactions with acid were examined. Both B2a/b and B4a contain a single pyrazolate moiety in the ligand, however, B4a additionally contains a hemilabile diethyl amine N. Nitrogen sites on

II II the ligand or Pt /Pt -CH3 could act as the site of protonation. If methane is not directly observed,

IV II either a Pt (H)(CH3) species or a ligand protonated Pt -CH3 is expected. In this case, conditions necessary for methane elimination from either proposed protonated product could then be studied.

We set out to investigate reactivity of acid with these complexes.

The protonation of B2a/b was first investigated. Upon the addition of 1 equivalent of HBF4 etherate to a diethyl ether solution of B2a/b, a precipitate was observed. When monitored in situ in

1 C6D6, no methane was observed by H NMR spectroscopy. Upon removal of C6D6 or ether and dissolution in THF-d8, one major product was observed containing an N-H resonance and platinum bound -S(CH3)2 and -CH3 ligands with no observable Pt-H resonance (expected upfield of 0 ppm).

H Me This species is postulated to be [Pt( NN )(S(CH3)2)CH3][BF4] (B6, Figure 2.04a); however, additional experiments such as solid state and/or NOSY experiments are required to assign the exact

Figure 2.04(a). Protonation of B2a or B2b with HBF4 etherate to form either B6a or B6b and (b). Protonation with HCl etherate to form methane after 2 added equiv.

structure (i.e. cis (B6a)/trans (B6b)). Complex B6 exhibits a methyl resonance with corresponding

2 1 2 JPt-H coupling of 79 Hz by H NMR spectroscopy. B6 displayed a smaller JPt-H coupling than

B2a/B2b (from 84 to 79 Hz). Additionally, no methane was observed in situ when 1 equiv. of HCl etherate was added to a CD2Cl2 solution of B2a/b; however, at least two Pt products are observed

t (based on 8 broad aromatic resonances and 2 sharp Bu resonances), along with liberation of S(CH3)2

33 by 1H NMR spectroscopy. Release of methane was not observed by 1H NMR spectroscopy until 2

1 total equivalents of HCl etherate was added to a CD2Cl2 solution of B2a/b. The resulting H NMR spectrum after a total of 3 equivalents of HCl etherate reveals at least 2 Pt products (based on 8

t aromatic resonances and 2 Bu resonances). Additionally, ESI-MS of the reaction mixture in CH3CN

H Me + suggests one of the main Pt products is Pt( NN )Cl(CH3CN) (m/z = 487.5).

H Me + Pt( NN )Cl(CH3CN) could form from solvent ligand exchange with weakly bound S(CH3)2. The formation of B6 after 1 equivalent of added HBF4 etherate to B2a/b and additionally, the liberation of methane after the addition of 2 equiv. of HCl etherate to B2a/b suggests the basicity of the PtII center and/or methyl ligand is less than that of the pyrazolate ligand. Only after the pyrazolate ligand is protonated does additional added acid liberate methane. This behavior is in contrast to that of

II 6,7 previous examples of bidentate ligated Pt -CH3 complexes, where addition of 1 equiv. of acid leads to a PtIV-H or methane release.

To determine if the pyrazolate N-H species (B6) is acidic enough to generate methane at elevated temperatures, thermolysis of B6 was attempted in THF-d8 in a sealed J. Young NMR tube.

At 100 °C, a reaction occurred. After 3 days of heating at 100 °C, no evidence of a PtIV-H or methane was observed by 1H NMR spectroscopy, although free dimethyl sulfide was present. Additionally, by 1H NMR spectroscopy, 4 Pt species were present at the end of the thermolysis reaction, based on

15 aromatic and 4 tBu resonances. It appears that methane elimination is disfavored over dimethyl sulfide release. The weakly binding nature of dimethyl sulfide appears to lead to dissociation which may somehow prevent C-H coupling. To address the lability of dimethyl sulfide, a tridentate ligand with a chelating diethyl amine moiety was targeted.

2.24 Release of methane from B4a under acidic conditions

Reaction of B4a with one equivalent of HBF4 etherate (Figure 2.05) in diethyl ether or benzene caused the precipitation of an orange solid. Upon isolation, the 1H and 13C{1H} NMR

Et Figure 2.0 3 Protonation of Pt(*NNN) CH3 (B4a) with HBF4 etherate to form H Et [Pt( NNN) CH3][BF4] (B7). Further Protonation of B7 with HCl etherate to form H Et [Pt( NNN) Cl][BF4] (B5a).

spectral data in CD2Cl2 were consistent with pyrazolate protonation and the formation of

H Et 1 [Pt( NNN) CH3][BF4] (B7). However, unlike earlier observations for the bidentate B6, in the H

NMR spectrum, no N-H resonance was observed when B7 was formed in C6D6 or CD2Cl2. When

IV 1 the reaction was monitored in situ in C6D6, no methane evolution or Pt -H was detected by H

NMR spectroscopy. The 19F{1H} NMR spectrum of B7 displayed two broad singlets in a 1:4 ratio,

- 10 11 28 consistent with an outer sphere BF4 and in accordance with the natural abundance of B: B.

2 The JPt-H coupling of the Pt-CH3 decreased slightly upon protonation from 79 Hz for B4a to 77

Hz for B7 in CD2Cl2. B7 was additionally characterized by high resolution ESI-MS (m/z =

495.2043). Under ESI-MS conditions, another species (with a mass consistent with Pt(*NNNEt)+) is present, presumably from decomposition of B7.

We determined that a second equivalent of acid is needed to generate methane. The addition of one equivalent of HBF4 etherate to a C6D6 solution of B7 generated methane at room temperature with no CDH3 formation. Thus, when two equivalents of acid are added to the unprotonated complex B4a, the first equivalent protonates the ligand. The second equivalent then protonates the Pt or the Pt-CH3 bond, leading to C-H coupling. The platinum product of this reaction displayed broad resonances in the 1H NMR spectrum and could not be identified.

However, when one equivalent HCl etherate, an acid with a coordinating conjugate base, was

H Et added to B4a, [Pt( NNN) Cl][BF4] (B5a, 69 % spectroscopic yield, Figure 2.05) was confirmed as the product. The formation of B5a in this reaction was confirmed by independent synthesis via protonation of the Pt-Cl (B5) with either HBF4 etherate or 2,6-dimethoxypyridinium tetrafluoroborate.

2.25 Release of methane from B7 and B4a by thermolysis

To determine if the ligand protonated complex B7 could release methane directly without an additional proton source, thermolysis studies were undertaken. Thermolysis of B7 was first

II attempted in acetonitrile. In contrast to ligand protonated N-H Pt -CH3 species which are reported to eliminate methane at low temperatures or upon warming to room temperature,14,15 no reaction occurred upon heating B7 in a sealed J. Young NMR tube up to 140 °C by 1H NMR spectroscopy.

Upon heating to 120 °C in the presence of H2O, a potential proton shuttle, B7 liberated a mixture of CH4 and CH3D and at least two new metal complexes formed, neither of which contained an N-

H resonance. Heating for longer times altered the ratios of the two Pt complexes and two broad 1H

NMR resonances increased in intensity around 6 ppm. The compound corresponding to these broad resonances was isolated. Upon analyzing by GC-MS, it was revealed that acetamide was being formed from solvent hydrolysis. At 0.1 mol % loading, 10 turnovers were observed. Several

Pt complexes have been shown to perform acetonitrile hydrolysis, with much milder conditions

Figure 2.0 4 Potential mechanism of acetonitrile hydrolysis.

37 and with higher TONs, so this reaction was not further pursued.29,30 A potential mechanism based on known Pt acetonitrile hydrolysis catalyst is presented in Figure 2.06.29

In contrast to experiments in acetonitrile, methane elimination (CH4 only) was observed when

B7 was subjected to elevated temperatures (100 °C) in C6D6 (Figure 2.07).. After three days, the disappearance of starting material was accompanied by the appearance of multiple Pt species (with no N-H resonances) as detected by 1H NMR spectroscopy, along with deposition of Pt black on the sides of the reaction vessel. Characterization of the reaction mixture by ESI-MS also revealed numerous species. The two major species exhibited an m/z of 565 and 481, consistent with the

D Et + Et + formation of [Pt( NNN )(C6D5)] and Pt(*NNN ) , respectively.

Figure 2.0 5 a) Thermolysis of B7 in C6D6 to form multiple Pt. b) Thermolysis of B4a in C6H6 to form B4b and multiple Pt.

Notably, it appears that the proton that combined with the Pt-CH3 group to form CH4 did not arise from the ligand-based N-H moiety. Thermolysis of the analogous B4a, which lacks a N-H functionality, also eliminated methane under similar conditions. After three days at 100 °C, 1H NMR spectroscopy in C6D6 revealed multiple products and redissolution of the reaction mixture in CD2Cl2 revealed at least two minor products and one major product. The reaction mixture exhibited an m/z

D Et + Et + of 565 and 481, consistent with the formation of [Pt( NNN )(C6D5)] and Pt(*NNN ) , respectively, under mass spectroscopy conditions. As the same products were observed by ESI-MS after the thermolysis of B7, this suggests the origin of the acidic proton is not from the pyrazolate N-

H moiety. Examination of the 2H NMR spectrum (Figure 2.08) of the thermolysis reaction mixture of B4a in C6D6 revealed two broad singlets at 2.80 ppm and 2.99 ppm, indicating deuteration of the

2 Figure 2.0 6 H NMR spectrum (107 MHz) in C6H6 of the thermolysis of B1 in C6D6. Referenced to additional CD3CN at 1.94 ppm. Deuterated methylene deuterons at 2.7 - 3.1 ppm. Aryl Pt-C6D5 deuterons at 6.8 – 7.6 ppm.

39 diethyl amine CH2 arm. Further confirmation of the major product of the thermolysis reaction was

Et achieved when thermolysis of B4a in benzene-H6 yielded Pt(*NNN) C6H5 (B4b, 25 % spectroscopic yield by 1H NMR spectroscopy, Figure 2.07). An authentic sample was added to the reaction mixture to confirm the identity.

These observations are consistent with a mechanism for the deuterium incorporation where, at elevated temperatures, dissociation of the hemilabile amine arm (Figure 2.09A) allows for

IV intramolecular C-H activation of the CH2 alkyl on the amine to generate a cyclometallated Pt species (Figure 2.09B).31 Reductive elimination of methane (Figure 2.09C), followed by oxidative

Figure 2.0 7 Potential mechanism for the thermolysis of B4a in C6D6 to form CH4 and Pt-C6D5.

40 addition of C6D6 solvent (Figure 2.09D) and subsequent reductive elimination of the amine alkyl

II with the deuteride on the metal (Figure 2.09E), generates the Pt -C6D5 product. Further evidence that the N-H functionality is not involved in the methane elimination was provided by the thermolysis

D Et of the deuterated version of B4a, Pt( NNN) CH3 (generated by stirring B4a in a D2O/THF (1:4)

1 mixture), which resulted in exclusive elimination of CH4 (i.e. no CDH3 was detected by H NMR spectroscopy).

2.26 Reactivity of Pt(*NNN)EtX Under Basic Conditions and in the Presence of Exogeneous

Ligands

Reactivity of B4a has previously been studied under acidic and neutral conditions (sect. 2.24 and 2.25) and it was determined that the metalated tridentate ligand is not always innocent in the observed reactions. The behavior of the metalated hemilabile ligand on Pt was additionally

t Figure 2. 10 Reaction of B5 with KO Bu in C6D6 to form B5b in situ.

investigated under basic conditions. Reaction of B4a in C6D6 with greater than 4 equivalents of NEt3 or KOtBu resulted in no reaction by 1H NMR spectroscopy. Reaction of B5, a Pt-Cl analogue of

t B4a, with 1 equiv of KO Bu in C6D6 led to a change from yellow to a bright red solution. A mixture

41 of new Pt species with broad 1H NMR resonances was observed. Conversion to a single new species, with signals downfield of B5 by 1H NMR spectroscopy, was achieved after addition of at least 1 more equiv. (total of at least 2 equiv.) of base. Complex B5b, presumed to be [K][Pt(*NN#N)EtCl]

(Figure 2.10, # denotes deprotonation of methylene arm) exhibited broad features by 1H NMR spectroscopy, including a reduction in the relative intensity of the methylene signal from 2H to 1H, as compared to the other 1H resonances for B5. Broad 1H NMR signals suggest an exchange process, perhaps with generated tert-butanol, whose signals migrated upfield as additional base was added.

B5b was not isolated. Additionally, reaction of B4a (Pt-CH3) with two or more equivalents of

t NaO Bu in CD3CN caused the resonance of the methylene arm at 4.33 ppm to disappear completely, with no shift in any other resonance, by 1H NMR spectroscopy (Figure 2.11). This suggests either potential deuteration at the methylene arm position from CD3CN solvent or a rapid exchange process at the methylene C-H position (Figure 2.11(a)), broadening the resonance to the baseline. Attempts to isolate this complex always lead to reappearance of B4a (i.e. the methylene proton in the 1H NMR

2 spectrum). However, examination of the reaction mixture by H NMR spectroscopy in C6H6 exhibits a broad singlet at 2.92 ppm, suggesting deuteration of the methylene arm (methylene CH2

1 resonances of B4a are exhibited at 3.01 ppm in the H NMR spectrum in C6D6). Both deprotonation experiments suggest the most acidic proton of B4a is not the CH2 proton of the diethyl amine arm, but instead is that of the methylene linker position. As methane elimination is believed to arise from

42 the CH2 of the ethyl groups on the amine arm, this suggests the most acidic proton on B4a does not facilitate methane elimination upon thermolysis.

Figure 2.1 1 (a) Proposed reaction resulting in disappearance of methylene proton resonance in the 1H NMR spectrum by deuteration. (b). 1H NMR spectrum of B4a in t CD3CN (bottom) and after addition of 2 equiv. of NaO Bu (top). The methylene proton resonance at 4.33 ppm is no longer present in the top spectrum after the addition of base.

Confirmation of hemilability of the diethyl amine arm to facilitate C-H activation was confirmed by a ligand exchange study. Addition of an excess of pyridine (over 10 equiv.) to B4a lead to the disappearance of starting material and appearance of two new Pt species by 1H NMR spectroscopy. One of the new complexes contained an unbound diethyl amine arm by 1H NMR spectroscopy, and numerous new resonances in the aromatic region by 1H NMR spectroscopy. It is noted that de-chelation of the diethyl amine arm is evident by examination of the 1H NMR spectrum where the methylene arm protons are no longer diastereotopic (Figure 2.12). Similar spectral changes

Figure 2.1 2 Reaction of B4a with exogeneous L ligands and how methylene protons change in the 1H NMR spectrum from equivalent to diastereotopic methylene protons are often used to determine chelation of hemilabile ligands.32 To obtain a single complex, two equivalents of a more strongly coordinating

1 Et . ligand, namely PPh3, were added to B4a in C6D6; Pt(κ -*NNN) (P(C5H6)3)2CH3 (B8a) was cleanly observed in situ. The formation of B8a was consistent by observations by 1H and 31P NMR

44 spectroscopy. In the 1H NMR spectrum, the diethyl amine arm is unbound (Figure 2.12) and

2 2 formation of broad singlet for the Pt-CH3 resonance with Pt satellites ( JPt-H = 58 Hz). The J is rather low for a Pt-CH3 moiety being trans to a pyridine ligand and suggests the Pt-CH3 group is trans to stronger pyrazolate donor.27 In the 31P{1H} NMR spectrum, appearance of a resonance at 25.19 ppm

1 ( JPt-P = 2882 Hz) was observed. Complex B8a could not be isolated due to the lability of PPh3, which was completely removed with reappearance of B4a after 5 iterative washes of diethyl ether.

Et Addition of excess P(CH3)3, a stronger phosphine donor, to a solution of Pt(*NNN) CH3 in C6D6

1 Et . 1 31 1 generated Pt(κ -*NNN) (P(CH3)3)2CH3 (B8b). B8b was isolated and characterized by H, P{ H}

13 1 2 and C{ H} NMR spectroscopy. Complex B8b contains a Pt-CH3 group with a JPt-H = 72 Hz at

2 1 0.60 ppm, which is notably larger than the JPt-H of B8a (55 Hz, 0.04 ppm) by H NMR spectroscopy,

33 due to reduced sterics on the phosphine ligand. Reaction of B8b with excess PPh3 resulted in no

1 ligand exchange by H NMR spectroscopy, confirming the tight binding nature of P(CH3)3.

2.3 Conclusion

In conclusion, we have reported new PtII pyrazolate complexes with protic functionality. A series of both PtII complexes were synthesized, supported by 5-(2-pyridyl)-3-tert-butylpyrazolate

(HNNH), 5-(6-methyl-2-pyridyl)-3-tert-butylpyrazolate (HNNMe) and 2-(5-tert-butylpyrazol-3-yl)-

6-(diethylaminomethyl)pyridine (HNNN)Et ligands. Metalation of HNNH resulted in formation of a

H H Me homoleptic complex, Pt(*NN )2 (B1), however, sterics from the methyl substituent in NN

Me prevented homoleptic formation and allowed formation of Pt(*NN )(S(CH3)2)CH3 (B2a/B2b). The

H Et II Et tridentate nature of ( NNN) allowed metalation with Pt to produce Pt(*NNN) CH3 (B4a),

Et Et H Et Pt(*NNN) C6H5 (B4b), and Pt(*NNN) Cl (B5). Lability of the diethyl amine moiety of NNN when ligated was demonstrated with addition of phosphine ligands to B4a to form Pt(κ1-

Et . 1 Et . *NNN) (P(C5H6)3)2CH3 (B8a) and Pt(κ -*NNN) (P(CH3)3)2CH3 (B8b). Conditions necessary for methane elimination were probed for B2a/B2b and B4a at room temperature and a series of acid addition experiments revealed methane elimination does not occur until all pyrazolate sites are protonated. Addition of 1 equivalent of HBF4 etherate to B2a/B2b or B4a formed

H Me H Et [Pt( NN )(S(CH3)2)CH3][BF4] (B6a/B6b) or [Pt( NNN) CH3][BF4] (B7), respectively. The

II ligand-based N-H is apparently not acidic enough to form methane from Pt -CH3 complexes

(B6a/B6b and B7) at room temperatures. Thermolysis of bidentate ligated B6a/B6b at 100 °C did not generate methane, yet, thermolysis of tridentate B7 did. It was determined that methane generation from the thermolysis of B7 was not dependent on the ligand-based N-H. Thermolysis of

B4a (which does not contain an N-H moiety) also eliminated methane. Activation of the benzene solvent to generate complex B4b was observed. Upon further examination of the thermolysis reaction, methane loss and benzene activation likely proceeded through oxidative addition of a C-H bond of the ethyl group on the NEt2 moiety, followed by reductive elimination, and not through a N-

H, methyl coupling. Additionally, reactivity of B4a under basic conditions was investigated and the results were consistent with deprotonation of the methylene CH2 moiety upon addition of base.

2.4 Experimental

2.41 General Experimental

All manipulations were carried out under nitrogen atmosphere using standard Schlenk and glovebox techniques unless otherwise noted. Deuterated solvents were purchased from Cambridge

Isotope Laboratories. Dry and O2 free tetrahydrofuran, benzene, pentane, methylene chloride, acetonitrile, and diethyl ether were obtained by means of a Grubbs-type solvent purification

34 system. THF-d8 and C6D6 were dried over sodium/benzophenone ketyl and were vacuum

46 transferred prior to use. Acetone-d6 and CD3CN were dried over activated 3 Å molecular sieves.

CD2Cl2 was dried over calcium hydride and vacuum transferred prior to use. PtCl2(S(CH3)2)2,

Pt(C6H5)2(S(CH3)2 and [Pt(CH3)2(µ-S(CH3)2)]2 were synthesized following literature preparations.35,36 All NMR spectra were obtained on a Bruker Avance 500 or Bruker Avance 400

MHz instrument. NMR spectra were recorded at 300 K. Chemical shifts are reported in units of parts per million (ppm) downfield of TMS and referenced against residual protonated solvent resonances (1H) and characteristic solvent resonances (13C). 31P{1H} NMR spectra were referenced

2 externally to H3PO4 (85%, 0 ppm) and H NMR spectra were referenced to the deuterium

19 1 resonance of extra added CD3CN (δ 1.94). F{ H} NMR spectra were referenced externally to

C6H5F (-113.15 ppm). NMR tubes fitted with a J-Young style Teflon valve were used to obtain inert atmosphere NMR data. The C, N, H elemental analyses were carried out at the CENTC

Elemental Analysis Facility at the University of Rochester. Accurate mass measurement analyses were conducted on a LCMS with electrospray ionization (ESI). Samples were taken up in a suitable solvent for analysis. The signals were mass measured against an internal lock mass reference of leucine enkephalin for ESI-LCMS. Waters software calibrates the instruments, and reports measurements, by use of neutral atomic masses. The mass of the electron is not included.

Nominal mass accuracy ESI-MS data were obtained by use of a Waters Acquity UPLC system equipped with a Waters TUV detector (254 nm) and a Waters SQD single quadrupole mass analyzer with electrospray ionization.

2.42 Synthesis, Characterization and Spectroscopic Data

Pt(*NN)2 (B1)

H H A J. Young NMR tube was charged with NN (10.0 mg, 0.0497 mmol) and [Pt(CH3)2(S(CH3)2)]2

(14.3 mg, 0.0248 mmol). CD2Cl2 was vacuum transferred to the J. Young tube and the sample was heated for 24 hrs at 60 °C and for 24 hrs at 100 °C. The J. Young NMR tube was cooled to -35 °C overnight and some solid was observed at the bottom of the NMR tube. The liquid was decanted and discarded, and the solid was dissolved in C6D6 and an NMR spectrum was recorded.

1 3 3 H NMR (500 MHz, CDCl3) δ 10.82 (2H, d, pyr, JH-H = 5.9 Hz), 7.84 (1H, t, pyr, JH-H = 7.9

3 t Hz), 7.57 (1H, d, pyr ( JH-H = 7.9 Hz)), 7.20 (1H, pyr, m) 6.51 (1H, pyz, s), 1.44 (18H, Bu, s).

1 Figure 2.13 H NMR spectrum (500 MHz) of B1 in C6D6.

Pt(*NN)(S(CH3)2)(CH3) (B2a or B2b)

H Me A 50 mL Schlenk flask was charged with the NN (69.5 mg, 0.324 mmol), Pt(S(CH3)2)2Me2

(94.0 mg, 0.162 mmol) and dissolved in THF (5 mL) under inert atmosphere. A reflux condenser was attached under positive nitrogen flow and the solution was heated at reflux for 90 minutes.

The solution was then cooled to room temperature and the volatiles were removed in vacuo to yield a yellow solid. B2 was confirmed as the major product by 1H NMR spectroscopy. The

49 following observations are noted. B2 was soluble in pentane (slightly), ether, DCM, benzene, THF,

CH3CN, and MeOH and decomposed when run through silica, neutral and basic alumina plugs.

Attempts to precipitate or crystallize out of concentrated pentane solutions did not produce cleaner

1H NMR spectra.

1 3 3 H NMR (500 MHz, Methanol-d4) δ 7.74 (1H, t, pyr, JH-H = 7.7 Hz), 7.55 (1H, d, pyr, JH-H = 7.8

3 3 Hz), 7.10 (1H , d, pyr, JH-H = 7.6 Hz), 6.54 (1H, pyz, s), 2.78 (3H, s, CH3), 2.49 (s, 6H, SMe2, JPt-

2 t H = 52.0 Hz), 1.39 (3H, s, Pt-Me, JPt-H = 83.5 Hz), 1.33 (9H, s, Bu).

1 Figure 2.14 H NMR spectrum (500 MHz) of B2 in methanol-d4.

Me Pt(*NN) (PPh3)(CH3) (B3)

A 20 mL scintillation vial was charged with the B2a/B2b (2.2 mg, 0.0045 mmol), PPh3 (1.2 mg,

0.0046 mmol), and a Teflon stir bar and dissolved in diethyl ether (1 mL) under inert atmosphere.

After vigorous stirring for 40 minutes, the solvent was removed under reduced pressure to yield a yellow solid. B3 was confirmed as the major product by 1H NMR spectroscopy. The following observations were noted. B3 was soluble in methanol, pentane and ether. Decomposition to multiple products was observed by 1H NMR spectroscopy when samples of B3 run through a diatomaceous earth plug.

1 3 H NMR (500 MHz, Methanol-d4) δ 7.81-7.65 (7H, m, PPh3), 7.62 (1H, d, pyr, JH-H = 7.3 Hz),

3 7.54-7.28 (9H , m, PPh3), 6.80 (1H, s, pyr, JH-H = 6.9 Hz), 6.63 (1H, s, pyz), 1.80 (3H, s, CH3),

t 3 2 31 1 1.36 (9H, s, Pt-Me, Bu), 0.97 (3H, d, JH-P = 5.0 Hz, JPt-H = 83.5 Hz). P{ H} NMR (162 MHz,

1 H Me + Methanol-d4) δ 18.22 ( JPt-P = 4186 Hz). ESI-MS: [Pt( NN )(CH3)(PPh3)] Theoretical Mass, m/z = 687.2; Observed Mass, m/z = 687.5

1 Figure 2.15 H NMR spectrum (500 MHz) of B3 in methanol-d4.

31 1 Figure 2.16 P{ H} NMR spectrum (500 MHz) of B3 in methanol-d4.

Et Synthesis of Pt(*NNN) CH3 (B4a)

A 20 mL scintillation vial was charged with a Teflon stir bar, 58.4 mg (0.204 mmol) of HNNNEt,

58.6 mg (0.102 mmol) [Pt(µ-S(CH3)2)(CH3)2]2 and 4 mL CH2Cl2. The reaction was vigorously stirred for 20 minutes. The solvent was removed in vacuo and the remaining solid was then triturated (2 x 2 mL) with pentane and further dried to yield an orange solid (80.1 mg, 79.2 %). 1H 3 3 NMR (CD2Cl2, 500 MHz): δ 7.83 (1H, t, pyr, JH-H = 7.9 Hz), 7.32 (1H, d, pyr, JH-H = 7.9 Hz), 3 4 7.08 (1H, s, pyr), 6.48 (1H, d, pyz, JH-H = 7.9 Hz), 4.23 (2H, t, CH2, JPt-H = 22 Hz), 3.20 (2H, 3 multiplet, CH2), 2.95 (2H, multiplet, CH2), 1.48 (6H, t, CH3, JH-H = 7.0 Hz), 1.34 (9H, s, CH3), 2 13 1 0.83 (3H, s, Pt-CH3, JPt-H = 78 Hz). C{ H} (CD2Cl2, 126 MHz): δ 161.50(s), 155.98(s), 151.65(s), 137.95(s), 116.43(s), 115.99(s), 100.68(s), 69.63(s), 59.28(s), 32.69(s), 31.20(s),

13.12(s), -15.26(s). Elemental Analysis: Anal. For C18H28N4Pt: Calc: C, 43.63; H, 5.70; N, 11.31. Found: C, 43.26; H, 5.57; N, 11.35.

1 Figure 2.17. H NMR spectrum (500 MHz) of B4a in CD2Cl2

13 1 Figure 2.18. C{ H} NMR spectrum (126 MHz) of B4a in CD2Cl2

Et Synthesis of Pt(*NNN) C6H5 (B4b)

H Et A J. Young NMR tube was charged with Pt(S(CH3)2)2(C6H5)2 (7.0 mg, 0.0147 mmol), NNN

(4.2 mg, 0.0147 mmol), and 0.5 mL CD2Cl2. The reaction was heated to 60 °C for 2.5 hours. After cooling, the yellow solution was concentrated to ~0.1 mL in a 20 mL scintillation vial in air.

Diethyl ether (2 mL) was then added until a yellow solid appeared, forming a suspension. The

54 supernatant was decanted, and the remaining solid washed with pentane (1 x 2 mL), and then dried

1 3 to obtain a pale yellow solid (6.6 mg, 81 %). H (400 MHz, CD2Cl2): δ 7.84 (1H, t, pyr-H ( JH-H =

3 3 3 8.0 Hz)), 7.53 (2H, d, pyr-H ( JH-H = 7.0 Hz, JPt-H = 39.4 Hz)), 7.37 (1H, d, pyr-H ( JH-H = 8.0

3 Hz)), 7.14 – 7.00 (3H, m, Ar-H and pyr-H overlapping), 6.92 (1H, t, Ar-H ( JH-H = 7.3 Hz)), 6.48

2 3 (1H, s, pyz-H), 4.31 (2H, s, CH2), 3.10 (2H, dq, CH2 ( JH-H = 10.4, JH-H = 7.1 Hz)), 2.79 (2H, dq,

2 3 3 13 1 CH2 ( JH-H = 13.7, JH-H = 7.0 Hz)), 1.53 (6H, CH3 ( JH-H = 7.1 Hz)), 1.27 (9H, s, CH3). C{ H}

(CD2Cl2, 126 MHz): δ 162.08(s), 157.33(s), 152.21(s), 151.61(s), 147.41(s), 138.70(s), 137.92(s),

127.02(s), 122.57(s) 116.38(s), 115.41(s), 101.13(s), 69.21(s), 59.97(s), 32.62(s), 31.15(s),

H Et + 13.27(s). ESI-MS: [Pt( NNN )C6H5] Theoretical Mass, m/z = 557.2176; Observed Mass, m/z =

557.2187.

1 Figure 2.19. H NMR spectrum (400 MHz) of B4b in CD2Cl2.

13 1 Figure 2.20. C{ H} NMR spectrum (101 MHz) of B4b in CD2Cl2.

Synthesis of Pt(*NNN)EtCl (B5)

A 20 mL scintillation vial was charged with 35.3 mg (0.123 mmol) of HNNNEt, 48.1 mg (0.123 mmol) Pt(S(CH3)2)2(Cl)2, triethyl amine (50 µL, 0.36 mmol) and 4 mL CH2Cl2. The reaction was vigorously stirred for 1 hour. The solvent was removed under reduced pressure, and the resulting solid was washed with diethyl ether (3 x 3 mL), and dried in-vacuo (41.9 mg, 66.0 %). 1H NMR

3 3 (CD2Cl2, 500 MHz): δ 7.97 (2H, vt, pyr, JH-H = 7.9 Hz, 7.8 Hz), 7.46 (1H, d, pyr, JH-H = 7.9 Hz),

3 7.17 (1H, d, pyr, JH-H = 7.8 Hz), 6.57 (1H, s, pyz), 4.39 (2H, s, CH2), 3.35 (2H, multiplet, CH2),

3 13 1 2.94 (2H, multiplet, CH2), 1.53 (6H, t, CH3, JH-H = 7.2 Hz), 1.32 (9H, s, CH3). C{ H} NMR

(CD2Cl2, 126 MHz): δ 161.28(s), 158.36(s), 152.82(s), 149.43(s), 138.43(s), 116.28(s), 114.95(s),

100.78(s), 67.53(s), 58.70(s), 30.53(s), 32.27(s), 12.22(s). Elemental Analysis: Anal. For

C17H25ClN4Pt: Calc: C, 39.58; H, 4.88; N, 10.86. Found: C, 39.38; H, 4.81; N, 10.63.

1 Figure 2.21 H NMR spectrum (500 MHz) of B5 in CD3CN

13 1 Figure 2.22 C{ H} NMR spectrum (126 MHz) of B5 in CD2Cl2

Synthesis of Pt(HNNN)EtCl (B5a)

A 20 mL scintillation vial was charged with 7.2 mg (0.014 mmol) B5, a Teflon stir bar, 3.2 mg

(0.014 mmol) 2,6-dimethoxypyridinium tetrafluoroborate and diethyl ether (3 mL). The suspension was stirred vigorously for 15 minutes. Diethyl ether (5 mL) was added to completely precipitate a very light yellow solid. The mother liquor was decanted, and the resulting solid was triturated with ether (4 mL) and washed with diethyl ether (2 x 3 mL). The solid was collected and

1 dried (8.1 mg, 96 % yield). H NMR (CD2Cl2, 500 MHz): δ 10.56 (1H, s, N-H), 8.29 (1H, t, pyr,

3 3 3 JH-H = 8.1 Hz)), 7.86 (1H, d, pyr, JH-H = 8.1 Hz), 7.79 (1H, d, pyr, JH-H = 8.1 Hz), 6.90 (1H, d,

4 4 2 3 pyz, JH-NH = 2.0 Hz), 4.69 (2H, t, CH2, JPt-H = 16.6 Hz), 3.42 (2H, dq, CH2, JH-H = 12.7, JH-H =

2 3 3 7.2 Hz), 3.06 (2H, dq, CH2, JH-H = 12.7, JH-H = 6.9 Hz), 1.57 (6H, t, CH3, JH-H = 7.1 Hz), 1.44

13 1 (9H, s, CH3), C{ H} (CD2Cl2, 126 MHz): δ 161.91 (s), 158.12 (s), 154.51 (s), 148.27 (s), 141.33

(s), 122.60 (s), 121.49 (s), 103.35 (s), 69.81 (s), 60.85 (s), 32.62 (s), 29.87 (s), 13.24 (s). 19F{1H}

H Et + NMR (CD2Cl2, 377 MHz): δ -151.68 (s), -151.73 (s). ESI-MS: [Pt( NNN) Cl] Theoretical Mass, m/z = 515.1473; Observed Mass, m/z = 515.1456.

1 Figure 2.23. H NMR spectrum (500 MHz) of B5a in CD2Cl2. A small amount of 2,6- dimethoxypyridinium tetrafluoroborate could not be removed and is observed at 7.85 (t), 6.49 (d) and 4.00 (s) ppm.

13 1 Figure 2.24. C{ H} NMR spectrum (126 MHz) of B5a in CD2Cl2. A small amount of 2,6- dimethoxypyridinium tetrafluoroborate could not be removed and is observed at 100.91 and

125.82 ppm.

19 1 Figure 2.25. F{ H} NMR spectrum (376 MHz) of B5a in CD2Cl2. A small amount of 2,6- dimethoxypyridinium tetrafluoroborate could not be removed and is observed at -148.85 (s) and -

148.91 (s).

In-situ Synthesis of [K][Pt(*NN#N)EtCl] (B5b)

A J. Young NMR tube was charged with 4.1 mg (0.0079 mmol) B5 and 0.4 mL C6D6 to form a yellow suspension. Addition of 2.1 mg (0.019 mmol) KOtBu resulted in a red solution. The

1 following H NMR spectrum was recorded: (C6D6, 500 MHz): δ 6.53 (1H, m, pyr), 6.27 (1H, s,

3 3 pyz), 5.86 (1H, d, pyr, JH-H = 7.5 Hz), 5.58 (1H, d, pyr, JH-H = 6.3 Hz), 3.47 (1H, s, C-H), 3.38

(2H, s, CH2), 2.28 (2H, s, CH2), 1.67 (6H, s, CH3), 1.35 (9H, s, CH3).

1 Figure 2.26. H NMR spectrum (700 MHz) of B5b in C6D6

H Me Synthesis of [Pt( NN) (S(CH3)2)(CH3)][BF4] (B6a or B6b)

A 20 mL scintillation vial was charged with B2 (39.0 mg,0.0802 mmol), a stir bar and dissolved in 3 mL of benzene. While stirring, 11 µL HBF4 etherate (0.080 mmol, 54 % in diethyl ether) was

62 added to precipitate an orange solid. The solvent was removed in vacuo and the resulting solid was washed with pentane (2 x 3 mL). A representative 1H NMR spectrum was obtained and is consistent with B6 as the major product.

1 H NMR (THF-d8, 500 MHz): δ 12.53 (1H, br, N-H), 7.96-8.06 (3H, m, pyr and pyz), 7.40 (1H, d,

4 3 pyr, JH-NH = 1.7 Hz), 2.84 (3H, s, CH3), 2.52 (6H, s, S(CH3)2, JPt-H = 61 Hz), 1.45 (9H, s, CH3),

3 1.29 (3H, s, CH3, JPt-H = 79 Hz).

1 Figure 2.27 H NMR spectrum (500 MHz) of B6 in THF-d8

H Et Synthesis of [Pt( NNN) CH3][BF4] (B7).

A 20 mL scintillation vial was charged with 19.2 mg (0.0387 mmol) B4a, a Teflon stir bar, and benzene (2 mL). The suspension was stirred vigorously while HBF4 etherate (5.0 µL, 0.0368 mmol, 54 % in diethyl ether) was added by syringe. The mixture was stirred for 5 minutes. Diethyl ether (10 mL) was added to completely precipitate out a yellow solid. The solid was filtered and washed with additional diethyl ether (2 x 5 mL). After the resulting solid was dissolved in minimal acetonitrile, 10 mL diethyl ether was added to form a suspension. The suspension was filtered and

1 the solid collected, and further dried under vacuum (18.8 mg, 83 % yield). H NMR (CD2Cl2, 400

3 3 3 MHz): δ 7.90 (1H, t, pyr, JH-H = 8.0 Hz), 7.41 (1H, d, pyr, JH-H = 8.0 Hz), 7.18 (1H, d, pyr, JH-H

4 2 = 7.9 Hz), 6.53 (1H, d, pyz)), 4.28 (2H, t, CH2, JPt-H = 24 Hz), 3.20 (2H, dq, CH2, JH-H = 12.5,

3 2 3 3 JH-H = 7.2 Hz), 2.94 (2H, dq CH2, JH-H = 12.5, JH-H = 6.9 Hz), 1.47 (6H, t, CH3, JH-H = 7.0 Hz),

2 13 1 1.34 (9H, s, CH3), 0.83 (3H, s, Pt-CH3, JPt-H = 77 Hz). C{ H} (CD2Cl2, 126 MHz): δ 160.65 (s),

156.31 (s), 152.60 (s), 150.64 (s), 137.98 (s), 117.31 (s), 117.24 (s), 101.18 (s), 69.88 (s), 59.56

3 19 1 (s), 32.64 (s), 30.92 (s), 13.21 (s, JPt-C = 32 Hz), -15.95. F{ H} NMR (CD2Cl2, 377 MHz): δ -

H Et + 152.2 (s), -152.3 (s). ESI-MS: [Pt( NNN) CH3] Theoretical Mass, m/z = 495.2019; Observed

Mass, m/z = 495.2043.

1 Figure 2.28. H NMR spectrum (400 MHz) of B7 in CD2Cl2

13 1 Figure 2.29. C{ H} NMR spectrum (126 MHz) of B7 in CD2Cl2

19 1 Figure 2.30. F{ H} NMR spectrum (376 MHz) of B7 in CD2Cl2

1 Et . In-situ Synthesis of Pt(κ -*NNN) (P(C5H6)3)2CH3 (B8a)

A J. Young NMR tube was charged with 5.0 mg (0.0079 mmol) B4a, 5.8 mg (0.022 mmol) PPh3 and C6D6 (0.4 mL) to form a yellow solution. Isolation of B8a was attempted by addition of 1 mL diethyl ether to the C6D6 solution to precipitate a yellow solid. The suspension was decanted and the resulting solid was washed with diethyl ether (2 x 1 mL) and resulted in reformation of B4a.

1 The following H NMR spectrum, in the presence of excess PPh3 was recorded: (C6D6, 500 MHz):

3 3 δ 7.82-7.69 (14H, m, Ar-H), 7.53 (1H, d, pyr, JH-H = 7.6 Hz), 7.47 (1H, d, pyr, JH-H = 7.6 Hz),

3 7.44-7.34 (13H, m, Ar-H)), 7.29 (1H, t, pyr, JH-H = 7.6 Hz), 7.08-6.96 (42H, m, Ar-H), 6.32 (1H,

3 s, pyz), 4.07 (2H, s, CH2), 2.61 (4H, q, CH2, ( JH-H = 7.6 Hz)), 1.37 (9H, s, CH3), 1.05 (6H, t, CH3,

3 3 2 31 1 JH-H = 7.6 Hz), 0.039 (3H, t, Pt-CH3, JH-P = 5.9 Hz, JPt-H = 55 Hz). P{ H} (C6D6, 202 MHz): δ

1 25.19 (Pt-PPh3, JPt-P = 3310 Hz), -5.30 (free PPh3).

1 Figure 2.31. H NMR spectrum (500 MHz) of B8a in C6D6

Figure 2.32. 31P{1H} NMR spectrum (162 Hz) of B8a. Resonance at -5.30 is noncoordinated PPh3.

1 Et Synthesis of Pt(κ -*NNN) (P(CH3)3)2CH3 (B8b)

A J. Young NMR tube was charged with 5.0 mg (0.0079 mmol) B4a and C6D6 (0.4 mL) to form a yellow suspension. An excess of P(CH3)3 (~0.01 mL) was vacuum transferred to the J. Young

NMR tube. The tube was sealed and was allowed to react for 20 mins. The volatiles were removed in vacuo, C6D6 was added and NMR spectra were recorded.

1 3 3 H (C6D6, 300 MHz): δ 8.75 (1H, d, pyr, JH-H = 7.4 Hz, 1H), 7.39 (1H, d, pyr, JH-H = 15.2 Hz),

3 3 7.39 (1H, t, pyr, JH-H = 15.2 Hz), 7.28 (1H, s, pyz), 3.91 (2H, s, CH2), 2.55 (4H, q, CH2, JH-H =

3 2 7.1 Hz), 1.74 (9H, s), 1.01 (6H, t, CH3, JH-H = 7.1 Hz), 0.84 (18H, vt, P(CH3)3, JH-P = 5.7 Hz,

2 3 2 3 JH-P = 5.7 Hz, JPt-H = 29.9 Hz, 0.60 (3H, t, Pt-CH3, JPt-H = 71.7 Hz, JH-P = 6.9 Hz). Virtual triplet simulated on MNova Spin Simulation to obtain coupling constants and are shown below

1 13 recorded H NMR spectrum. C NMR (176 MHz, C6D6) δ 163.12 (s), 149.70 (s), 135.73 (s),

128.36 (s), 118.61 (s), 117.45 (s), 101.40 (s), 60.69 (s), 47.93 (s), 32.58 (s), 32.01 (s), 12.60 (s),

2 31 1 12.51 (s), 12.40 (s), -27.52 (Pt-CH3, t, ( JP-H = 14.9 Hz)). P{ H} (C6D6, 202 MHz): δ -18.91

2 1 (Pt-P(CH3)3, JP-P = 7.4 Hz, JPt-P = 2886 Hz).

1 Figure 2.33. H NMR spectrum (300 MHz) of B8b in C6D6

1 Figure 2.34. Simulated H NMR spectrum of Pt-CH3 resonance of B8b in C6D6 in top spectrum

1 (1). Figure 2.34-2. Actual H NMR spectrum of Pt-CH3 resonance in C6D6.

13 1 Figure 2.35. C{ H} NMR spectrum (300 MHz) of B8b in C6D6

31 1 Figure 2.36. P{ H} NMR spectrum (162 MHz) of B8b in C6D6

2.43 X-ray Crystallography General Information

Complexes B4a: X-ray intensity data were collected on a Bruker APEXIII D8QUEST37 CMOS area detector, both employing graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) at 100(1)

K. Preliminary in indexing was performed from a series of twenty-four 0.5° rotation frames with exposures of 10 seconds. Rotation frames were integrated using SAINT38, producing a listing of unaveraged F2 and σ(F2) values. The intensity data were corrected for Lorentz and polarization effects and for absorption using SADABS.39 The structure was solved by direct methods –

ShelXT.40 Refinement was by full-matrix least squares based on F2 using SHELXL-2018.41 All

2 2 reflections were used during refinement. The weighting scheme used was w=1/[σ (Fo )+

2 2 2 (0.0612P) + 1.0285P] where P = (Fo + 2Fc )/3. Non-hydrogen atoms were refined anisotropically and hydrogen atoms were refined using a riding model.

Complexes B4b and B5: X-ray intensity data were collected at -173 °C on a Bruker APEX II single crystal X-ray diffractometer, Mo-radiation. The data was integrated and scaled using SAINT,

SADABS within the APEX2 software package by Bruker.42 Solution by direct methods (SHELXS,

SIR97)43,44 produced a complete heavy atom phasing model consistent with the proposed structure.

The structure was completed by difference Fourier synthesis with SHELXL97.45,46 Scattering factors are from Waasmair and Kirfel.47 Hydrogen atoms were placed in geometrically idealised positions and constrained to ride on their parent atoms with C---H distances in the range 0.95-1.00

Angstrom. Isotropic thermal parameters Ueq were fixed such that they were 1.2Ueq of their parent atom Ueq for CH's and 1.5Ueq of their parent atom Ueq in case of methyl groups. All non-hydrogen atoms were refined anisotropically by full-matrix least-squares.

Complex B4b B5 B4a

Emperical Formula C23 H24 D9 N4 Pt C17 H25 Cl N4 Pt C19H30Cl2N4Pt Formula weight 566.65 515.95 580.46 Temperature (K) 100(2) 100(2) 100 Wavelength (Å) 0.71073 0.71073 0.71073 Crystal System Tetragonal Monoclinic monoclinic

Space group I 41/a C 21/n P21/c Unit cell axes (Å) a = 19.9089(8), b = 19.9089(8), c =31.220(3) a = 14.9564(11), b = 11.0974(7), c = 11.1395(8) a =23.0153(10), b = 10.8687(5), c = 26.1940(11) Unit cell angles (°) a= 90, β=90, γ = 90 a= 90, β= 101.880(4), γ = 90 92.569(2) Volume (Å3) 12374.6(13) 1809.3(2) 6545.7(5) Z 16 4 12 Demsity (mg/m3), calc. 1.217 1.894 1.767 Absorption coeff. (mm-1) 4.546 7.907 6.686 F(000) 4384 1000 3408 Crystal size (mm3) 0.04 x 0.02 x 0.02 0.08 x 0.06 x 0.01 0.32 x 0.09 x 0.05 Theta range for data collection (°) 2.05 to 25.40 2.09 to 26.56 5.942 to 55.112 Index ranges -23<=h<=23, -23<=k<=23, -37<=l<=37 -18<=h<=18, -13<=k<=13, -13<=l<=13 -29 ≤ h ≤ 29, -14 ≤ k ≤ 13, -34 ≤ l ≤ 34 Reflections collected 92722 69933 156801 Independent reflections, R(int) 5676 [R(int) = 0.0751] 3740 [R(int) = 0.0967] 15082[R(int) = 0.0471] Completeness to theta (% ) 99.8 100 99.8 Max. and min. transmission 0.9349 and 0.8391 0.9251 and 0.5703 0.4984 and 0.7456 Refinement Method Full-matrix least-squares on F2 Full-matrix least-squares on F2 Full-matrix least-squares on F2 Data/restraints/parameters 5676/15/260 3740/24/226 15082/72/751 Goodness-of-fit on F2 1.085 1.045 1.087

Final R indices [I>2sigma(I)] R1 = 0.0567, wR2 = 0.1085 R1 = 0.0301, wR2 = 0.0517 R1 = 0.0224, wR2 = 0.0454

R indices (all data) R1 = 0.1182, wR2 = 0.1471 R1 = 0.0497, wR2 = 0.0574 R1 = 0.0328, wR2 = 0.0486 -3 Largest diff. peak and hole (e.A ) 2.452 and -1.097 1.275 and -1.529 0.69 and -1.67

Table 2.1 Parameters for X-Ray Structures in Chapter 2

2.5 Notes to Chapter 2

(1) Shilov, A. E.; Shul, G. B. Chem. Rev. 1997, 2665, 2879–2932.

(2) Gunsalus, N. J.; Koppaka, A.; Park, S. H.; Bischof, S. M.; Hashiguchi, B. G.; Periana, R.

A. Chem. Rev. 2017, 117, 8521–8573.

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Chapter 3

Synthesis of Pyrazolate Supported Tridentate PtII Alkyl Complexes and Reactivity with

Electrophiles

3.1 Introduction

Oxidative addition is a fundamental reaction in organometallic chemistry. Understanding

+ + how d8 transition metals undergo oxidative addition of E-X (E = CH3 , H ) substrates has led to advances in cross coupling, carbonylation and N-alkylation reactions, to name a few.1,2 Electron rich precious metals form stronger M-E bonds than their first row counterparts and are often used to study catalytic systems which go through oxidative addition pathways.3 As Pt can form solution-stable complexes in various oxidation states (0/II/IV), it is often used to explore these 2-electron metal oxidation reactions.

II Bidentate ligand-supported Pt (CH3)2 complexes are susceptible to oxidative addition of EX substrates.4 The oxidative addition of E-X to square planar Pt(II) generally proceeds through an associative mechanism involving several steps (Scheme 3.01). In the first step, E-X undergoes

IV + nucleophilic attack by Pt(II) to form a five-coordinate [L2Pt (CH3)2E] cationic intermediate. In the

- IV second step, X (or solvent) coordinates to form an octahedral L2Pt (CH3)2(E)(X/solv.) complex 77 Portions of this chapter have been adapted from: Zahora, B. A.; Gau, M. R.; Goldberg, K. I. In Preparation II Scheme 3.01. Generalized reaction of L2Pt (CH3)2 with HX and CH3X

(Scheme 3.01). The oxidized PtIV product can often be thermodynamically unstable. In this case, the

II CH3-E (E = CH3, H) coupled product and the resulting Pt product after reductive elimination occurs are often observed (Scheme 3.01, Step 3). This is due, in part, to the thermodynamics of such a system. To start, the entropy of oxidative addition of a C-H moiety requires ca. 10 kcal/mol that

5 needs to be overcome. Additionally, the formation of both CH3-E bonds [ca. ~100 kcal/mol (C-H bond), ca. ~90 kcal/mol (C-C bond)] and the resulting PtII-X/solv bond [ca. ~65 kcal/mol (PtII-Cl

IV IV bond)] is often greater than the newly formed Pt -CH3 (ca. ~35 kcal/mol) or Pt -H (ca. ~75 kcal/mol) bonds once PtII to PtIV oxidation occurs.3,5–7

For example, HCl addition to a 2,9-dimethyl-l,10-phenanthroline (dmphen) ligated

II IV Pt (CH3)2 complex formed isolatable 6-coordinate Pt (dmphen)(CH3)2(H)(Cl) (Step 1 and 2 combined, Scheme 3.01), which slowly reductively eliminated methane in solution to form

II 8 Pt (dmphen)(CH3)(Cl) (Step 3, Scheme 3.01). Whether methane liberation occurred from 5-

78 IV + IV coordinate [Pt (dmphen)(CH3)2(H)] or 6-coordinate Pt (dmphen)(CH3)2(H)(Cl) was not studied, however, most studies of reductive elimination of alkanes from PtIV alkyl hydride complexes support that the concerted C-H bond forming step occurs from a five-coordinate metal species.9 Similar to

II HCl, addition of CH3I to a bipyridine (bpy) ligated Pt (CH3)2 complex formed

IV [Pt (bpy)(CH3)3(solv)][I] in polar solvents (CD3CN and acetone-d6) at reduced temperatures (-20

10 IV °C). Warming to room temperature resulted in formation of the stable Pt (bpy)(CH3)3(I) (Scheme

IV 3.01, bottom pathway). While thermolysis of Pt (bpy)(CH3)3(I) was not studied, thermolysis of a

IV similar 6-coordinate 1,2-bis(diphenylphosphino)ethane (dppe) ligated Pt (dppe)(CH3)3(I) formed

II 11 ethane and Pt (dppe)(CH3)(I). This C-C reductive elimination was proposed to proceed via a five-

+ + II II coordinate intermediate. For both additions of E-X, E = H and CH3 , to Pt , the Pt center acts as the initial nucleophile, with subsequent coordination of X- anion to form the corresponding 6-

IV coordinate Pt species (Scheme 3.01, Step 1 and 2). Methane or ethane is then released (CH3-E reductive elimination; Scheme 3.01, Step 3) from a 5-coordinate PtIV oxidized product (after X- dissociation) to form the PtII product.

+ + II Although addition of an E-X (E = H , CH3 ) electrophiles to nitrogen ligated Pt (CH3)2 species has generally been thought to be initiated by nucleophilic attack by the metal (Scheme 3.01,

Step 1), several examples have been reported for complexes which bear an additional basic/nucleophilic donor site on the ligand.12–14 In this case, it would be possible for ligand donor sites, such as N, to act as the nucleophile in attacking an electrophilic reagent and oxidation to PtIV would not occur. For example, HCl addition to a

II dimethyl((pyridinylmethylene)amino)(ethyl/propyl)amine (DMEP/DMPP) ligated Pt (CH3)2

II H H complex formed a protonated [Pt ( DMEP/ DMPP)(CH3)2][Cl] upon the addition of the first equivalent of acid (Figure 3.01a).12 Once the ligand is protonated, the next equivalent of HCl to

79 II H H Pt ( DMEP/ DMPP)(CH3)2 reacted at the metal to form

IV H H [Pt ( DMEP/ DMPP)(CH3)2(H)(Cl)][Cl]. Selective protonation at the ligand with the first

II H H equivalent of HCl was due to stability of protonated Pt ( DMEP/ DMPP)(CH3)2 over

IV Pt (DMEP/DMPP)(CH3)2(H)(Cl), which was not observed. (A more in-depth analysis of this protonation was investigated in Chapter 2 of this thesis.)

Figure 3.0 1 (a) Reaction of Pt(DMEP/DMPP)(CH3)2 with HCl. (b) Reaction of Pt(bipym)(CH3)2 with CH3I. (c) Computational predicted products for reaction of Pt(cbipy)(CH3)2 with CH3I.

+ II In contrast to H addition, bipyrimidine (bipym) ligated Pt (CH3)2 undergoes oxidative

IV 13 addition with methyl iodide at the metal to form Pt (bipym)(CH3)3(I) (Figure 3.01b). Addition of

N-methylation was not observed. Furthermore, cyclometalated bipyridine (cbipy) ligated

80 II II Pt (CH3)(PPh3) contains a similar structured complex to Pt (bipym)(CH3)2, yet a ligand-based N is unbound and in the para position to the cyclometalated aryl ring (Figure 3.01c, initial complex).14

II Computational evidence reveals a 24 kcal preference of N-methylation over Pt oxidation upon CH3I addition, although they were unable to independently synthesize the appropriate complexes experimentally in the study. Preference for ligand N-methylation over PtII oxidation was suggested to be due to the steric accessibility of the ligand-centered N.14 As seen in Figure 3.01b, the N in

II Pt(bipym)(CH3)2 is more sterically crowded than Pt (cbipy)(CH3)2 in Figure 3.01c, which would support reactivity at PtII instead of the ligand N. These two examples are notable, as each ligand contains an additional unbound ligand N atom which can either act as a nucleophile or can be

+ II IV innocent. While in Figure 3.01b, addition of CH3 electrophile occurs first at Pt to form Pt species despite a potential nucleophilic N on the ligand, computational evidence has suggested N- methylation might be possible if a sterically accessible ligand N is present (Figure 3.01c).

II IV While there are many examples of Pt oxidation with HCl and/or CH3I to Pt , conditions exist which can result in selective protonation or N-methylation, respectively. We set out to explore

Scheme 3.02. Metalation plan to form Pt-CH3 complexes. Reactivity of Pt-CH3 with EX (E = + + CH3 or H ).

81 metalation and subsequent reactivity of 5-tert-butyl-1,3-bis(pyrazol-3-yl)pyridine (HNNNH) and 5- tert-butyl-1,3-bis(pyrazol-3-yl)benzene ligand (HNCNH)R (R = H, tBu). Both ligands contain an additional potential reactive ligand-based N (Scheme 3.02). Once metalated, pyrazolate ligated PtII-

CH3 species were targeted and experiments were conducted to determine whether the ligand can

II participate in electrophile addition. Electrophile addition (EX) to pyrazolate ligated Pt -CH3 will either occur at PtII to form a PtIV(E)(X) species or will react with the available ligand-based N atom to form a N-E ligated [PtII][X] species.

3.2 Results and Discussion

3.21 Preparation of HNNNH Ligated Complexes

Chelation of multidentate pyrazolate ligands to PtII has been observed in protic solvents.15

H H Similarly, the addition of ( NNN ) to Pt(S(CH3))2Cl2 in refluxing methanol afforded

[Pt(HNNNH)Cl][Cl] (C1, 78 % isolated yield, Figure 3.02). 1H and 13C{1H} NMR spectroscopy

82 Figure 3.0 2 Metalation of HNNNH to PtII to form [Pt(HNNNH)Cl][Cl] (C1) and H H subsequent reactions to form [Pt( NNN )Cl][BF4] (C2), and H H [Pt( NNN )NCCH3][(BF4)2] (C3). Thermal ellipsoid plots (50% probability) of C2 and C3 are shown. Selected H-atoms and 1 molecule of acetylacetone in C2 omitted for clarity. Selected bond lengths for C2: Pt(1)-Cl(1) 2.296(3) Å, H(1)-N(11) 2.01 Å, H(5)-O(1) 1.850 Å, H(1b)-Cl(1) 2.491 Å, C3: H(5)-F(5) 1.826 Å. indicate that C1 contains a symmetrical ligand environment. Even under rigorously dry deuterated

1 solvents, including C6D6, acetone-d6 or THF-d8, no N-H signal was observed in the H NMR spectra.

H H An anion exchange of C1 with NaBF4 in CH3CN gave [Pt( NNN )Cl][BF4] (C2) in 78 % isolated yield (Figure 3.02). The 1H and 13C{1H} NMR spectral data for C2 are also consistent with a

1 symmetrical ligand environment. Compared to the H NMR spectrum of C1 in CD3OD, all resonances were shifted upfield. Additionally, a broad N-H resonance at 12.07 ppm was apparent when dissolved in dry CD3CN. Two broad singlets at -154.47 and -154.52 ppm, in a 1:4 ratio, were

19 1 - observed by F{ H} NMR spectroscopy. These signals are typical for an outer-sphere BF4 anion,

83 in accordance with the natural abundance of 10B : 11B.16 Complex C2 was also characterized in the solid state (Figure 3.02) and exhibits several hydrogen bond interactions (indicated by dashed lines in Figure 3.02) between the ligand N-H moieties, acetonitrile solvent, water and the chloride ligand.

Addition of two equiv. of AgBF4 to a CH3CN solution of C1 resulted in abstraction of both

H H chlorides to form [Pt( NNN )NCCH3][(BF4)2] (C3) in 88 % isolated yield (Figure 3.02). Similar to

C1 and C2, complex C3 has two protonated ligand-based pyrazolates. The resonance for these N-H protons appears at 12.92 ppm in the 1H NMR spectrum. The bound acetonitrile is seen in the solid- state structure of C3 (Figure 3.02) and can additionally be observed by 1H NMR spectroscopy at

2.94 ppm in CD2Cl2 solvent. Notably, in the solid state, both ligand-based N-H’s exhibit hydrogen

- bonding to the BF4 anions (1.826 Å). Hydrogen bonding in solution likely contributes to the downfield location of the N-H resonance in the 1H NMR spectrum.

3.22 Preparation of HNCNH Ligated Complexes

While metalation of tridentate NNN based ligands was performed under mild conditions, metalation of tridentate ligands requiring Ar-H activation to PtII often requires harsher conditions. It has been performed in refluxing glacial acetic acid (ca. 118 °C) and proceeds by electrophilic aromatic substitution (see chapter 1.3).17 Addition of 1 equiv. (HNCNH)R (R = H, tBu) to 1 equiv. of

H H R K2PtCl4 in an acetic acid : water (95 : 5) mixture in air affords Pt( NCN ) Cl (Figure 3.03) after two days at reflux. Formation of Pt(HNCNH)RCl is retarded when no water was added and took an additional 5 days, along with formation of an unidentified side product. However, when significantly

84 Figure 3.0 3 Metalation of HNCNH to PtII to form Pt(HNCNH)R’Cl (C4) and reactivity with AgBF4 to form C4b. Thermal ellipsoid plot (50% probability) of C4 and selected H-atoms omitted for clarity in the solid-state structure. Selected bond lengths for C4: O(1)-H(2) 2.144 C4 Å, Pt(1)-Cl(1) 2.421 Å, Cl(1)-H(1) 2.32 Å, C(3)- Pt(1) 1.932 Å.

more water was added (50 % water : 50 % acetic acid), significant formation of Pt0 was observed.

When R’ = H, Pt(HNCNH)HCl (C4, 73 % isolated yield) was formed. However, this reaction was found to be irreproducible and attempts to isolate C4 resulted in a mixture of products as evidenced by 1H NMR spectroscopy and ESI-MS. Attempts to address the experimental irreproducibility through altering starting material stoichiometry and performing the reaction under inert atmosphere and/or in the absence of ambient light, were targeted, yet ultimately unsuccessful. While the origin of the irreproducibility was not confirmed, one potential explanation is pyridyl ligand Ar-H activation, facilitated by labile protonated pyrazolates.18–20 Although no Pt-H resonances were observed by 1H NMR spectroscopy, rollover C-H activation of pyrazolate based ligands and pyridyl

20,21 ligands has been reported for square planar d8 complexes. Characterization of isolated C4 confirmed a symmetrical ligand environment by both 1H NMR spectroscopy and in the solid state

85 (Figure 3.03). In the solid state, two hydrogen bond interactions are observed between the ligand N-

H and a H2O molecule, and the water molecule and the Pt-Cl moiety (2.144 Å, and 2.32 Å, respectively). Additionally, the stronger trans-influence of the aryl moiety of C4 compared to the pyridyl moiety of C1 can be observed in the Pt-Cl bond distance (2.421 Å vs 2.289(4) Å respectively).

When R’ = tBu, a single product was isolated at the end of the reaction, Pt(HNCNH)tBuCl (C5,

88 % isolated yield). By 1H NMR spectroscopy, C5 is symmetrical in solution and contains 2 aryl resonances, 2 tBu resonances and a far downfield N-H resonance. A single side product was sometimes observed by 1H NMR spectroscopy, which could not be separated from C5. The side product was additionally symmetric, yet by ESI-MS in CH3CN, the mixture of products produced

H H + only one peak (m/z = 557.2, corresponding to Pt( NCN )NCCH3 ). The side product appears to be similar to C5 by NMR spectroscopy and ESI-MS and suggests another form of the metalated

(HNCNH)tBu to Pt. At most, two products were observed from the metalation attempts and we hypothesize steric bulk from the central tBu moiety of the ligand prevented both the irreproducibility and large number of products observed in metalation of HNCNH.

Elimination of the Cl- ligand through dehydrohalogenation was next attempted. Addition of two equivalents of base (and in some cases excess base) to Pt(NCN)tBuCl resulted in numerous

1 t species by H NMR spectroscopy. Bases investigated included NEt3, NaH, KO Bu, C9H16N2

(DBU), C10H6(NMe2)2 (proton sponge), and LiN(C3H7)2. None of the bases resulted in clean

1 H H tBu reactivity by H NMR spectroscopy. Additionally, when a 3:1 mixture of NaBH4 : Pt( NCN ) Cl in MeOH was stirred for 30 minutes, a mixture of products was observed by 1H NMR spectroscopy. Using material from this crude reaction mixture, X-ray quality crystals were grown

tBu H H tBu from slow evaporation of a pentane solution and revealed the complex Pt4(*NCN*) ( NCN ) 3

86 H H Figure 3.0 4 Thermal ellipsoid plots (50% probability) of Pt4(*NCN*)( NCN )3. 2 molecules of pentane, 4 molecules of water and H-atoms omitted for clarity. Selected H H bond lengths for Pt4(*NCN*)( NCN )3: Pt(2)-C(33) 1.959(7) Å, N(5)-Pt(1) 2.129(6) Å, N(8)-Pt(4) 2.182(6) Å, Pt(2)-Pt(3) 2.954 Å

(Figure 3.04). This shows the Cl- ligand was successfully removed, and a tetranuclear Pt species was formed with a bridging pyrazolate ligand. Multinuclear pyrazolate species have been observed throughout the literature and often occurs when the N-H moiety becomes deprotonated.18,19,22,23

Additionally, numerous N-H moieties in the solid-state structure were still present. However, dissolution of the crystals in MeOD gave at least one product, which was not representative of the bulk sample by 1H NMR spectroscopy when the deprotonation reaction was performed. As basic solutions of C5 appeared to form multiple products and/or multinuclear species, a more forcing method of Cl- removal was next attempted.

Similar to C1, the Cl- ligand can be removed to form a single product by 1H NMR spectroscopy when 1 equivalent of AgBF4 was added to a suspension of C4 in C6D6 (Figure 3.03).

Two species initially formed, evidenced by 1H NMR spectroscopy; however, after three days, the species converged to a single species by 1H NMR spectroscopy. This unknown species (C4b)

87 appeared symmetrical in solution and an N-H resonance was observed at 12.02 ppm. No

1 decomposition of C4b was observed in a C6D6 solution over 24 hours by H NMR spectroscopy. It is unclear what ligand replaced the abstracted Cl-, however, as neutral C4 had poor solubility in benzene and C4b is fully soluble, it is unlikely the resulting product is cationic. Examination by ESI-

H H + MS in CH3CN revealed a single peak at m/z = 557.2 (corresponding to [Pt( NCN )NCCH3] ) and suggests a ligand easily displaced under ESI-MS conditions. When in a C6H6 solution, a single deuteride resonance was observed by 2H NMR spectroscopy at 1.49 ppm with no Pt satellites

19 1 (referenced to additional C6D6). Additionally, no resonance was observed by F{ H} NMR spectroscopy. A solid-state structure would be required to elucidate the identity of the product.

Additionally, performing the chloride abstraction in coordinating solvent (e.g. CH3CN) would aid in characterization.

3.23 Synthesis of *NNN* Ligated Pt-Alkyl Species

The reaction of excess CH3Li (6 equiv.) with C1 in THF, followed by quenching with water

(3-50 equiv.), afforded [Li2Cl][Pt(*NNN*)CH3](THF)4 (C6) in 69 % isolated yield (Figure 3.05).

1H and 13C{13H} NMR spectroscopy indicated C6 is symmetrical in solution. A characteristic Pt-

88 Figure 3.0 5 Reactivity of C1 with CH3Li to form [Li2Cl][Pt(*NNN*)CH3] (C6) and in the presence of CH3CN to form [Li(THF)]2[Pt(*NNN*)CH2CN]2 (C7). Cation exchange reaction of C6 with PPNCl to form [PPN][Pt(*NNN*)CH3 (C8). Thermal ellipsoid plots (50 % probability) of C6, C7 and C8. Solvents (C6 – 4 molecules of THF, C7 – 2 molecules of pentane, C8 – 2 molecules of THF and 1 molecule of pentane), PPN cation in C8, and H-atoms are omitted for clarity. Selected bond lengths for C6: C(1)-N(1) 1.358(9) Å, C(3)-N(4) 1.368(9) Å, Li(1)-N(1) 2.01(1) Å, Li(1)-C(20) 2.88(2) Å, C(1)C(2) 1.397(6) Å, C(2)C(3) 1.388(6), Pt(1)-C(20) 2.053(6) Å. C7: N(4)-Li(1) 1.94(4) Å, Li(1)- N(6) 2.26(4) Å, C(21)-N(6) 1.15(2) Å, Pt(1)-C(20) 2.05(2) Å, Pt(1)-N(3) 1.96 Å. C8: C(1)N(1) 1.357(6), C(3)N(2) 1.376(5), C(1)C(2) 1.39(1) Å, C(2)C(3) 1.39(1) Å, Pt(1)- C(20) 2.061(3) Å.

2 CH3 resonance was observed at 1.11 ppm ( JPt-H = 79 Hz) in THF-d8. Examination of the solid-state

89 structure of C6 (Figure 3.05) revealed a pyrazolate Li-Cl-Li interaction with THF solvated Li cations. Further evidence for Li coordination is the broad singlet at -1.20 ppm in the 7Li{1H} NMR spectrum of C6 in acetone-d6. Dissolution of C6 in CD3CN or acetone-d6 results in the liberation of

THF by 1H NMR spectroscopy. There have been similar reports of first row transition metals with

Li-Cl pyrazolate adducts24 and these examples help demonstrate the ability of ligand-based pyrazolate N to additionally act as a Lewis base. The Li-N distance of C6 is 2.01(1) Å and is slightly long compared to other inner-sphere Li ion-pyrazolate pairing distances with other metals (M = Co,

Ni, Fe).24,25 In fact, a much shorter Li-N distance (1.678 Å) was observed in a pyrazolate ligated CoI complex.24 This was postulated to be due in part from further interaction of Li+ with the CoI-Cl ligand.

In the presence of trace amounts of acetonitrile, reaction of C1 with excess CH3Li in THF prevents formation of C6 as the final product and forms [Li(THF)]2[Pt(*NNN*)CH2CN]2 (C7,

Figure 3.05) in 66 % isolated yield. Formation of C7 is not an intermediate in the formation of C6

1 as no reaction was observed by H NMR spectroscopy when 3 equiv. CH3Li were added to a THF-

1 13 1 7 1 d8 solution of C7. C7 was characterized by H, C{ H}, and Li{ H} NMR spectroscopy and X-ray crystallography. 1H, 13C{13H} and 7Li{1H} NMR spectroscopy indicated complex C7 is symmetrical

2 in solution. A characteristic Pt-CH2CN resonance was observed at 2.50 ppm ( JPt-H = 110 Hz) in acetone-d6. Examination of the solid-state structure revealed dimeric formation through the bridging and bent CH2CN ligand and Li cation.

Removal of the LiClLi+ cation by treatment of C6 with 1 equiv. bis(triphenylphosphine)iminium chloride (PPNCl) yields [PPN][Pt(*NNN*)CH3] (C8) in (Figure

3.05). The solid-state structure (Figure 3.05), the lack of a resonance in the 7Li{1H} NMR spectrum, and the presence of a resonance in the 31P{1H} spectrum, confirms the pyrazolate ligand is no longer

90 involved in an inner sphere cation interaction with lithium. Complex C8 appears symmetrical in

1 13 1 2 solution by H and C{ H} spectroscopy with a characteristic Pt-CH3 resonance at 1.11 ppm ( JPt-H

= 82 Hz) in CD3CN.

Interestingly, reaction of a THF solution of CH3MgCl (4.2 equiv.) with 1 equiv. of C1 in benzene, followed by quenching with water (3 equiv.) did not result in a Pt-CH3 with an inner sphere

H H Mg, but resulted in the generation of [Pt( NNN )CH3][Cl], (C9). Isolation of C9 in the absence of the MgX2 salts formed (hypothesized as present from THF impurity present in spectral data) has not been possible, yet characterization by 1H NMR spectroscopy indicates that C9 is symmetrical in solution. A broad N-H resonance was observed downfield at 13.3 ppm and a characteristic Pt-CH3

2 1 resonance was found at 1.24 ppm ( JPt-H = 78 Hz) in acetone-d6 in the H NMR spectrum. C9 was also characterized by X-ray crystallography. In the solid state (Figure 3.07b), a chloride anion is also present at a hydrogen bonding distance (2.270 Å) from the N-H on the ligand. Reaction of C9 with

F H H F NaBAr 24 (0.9 equiv) yielded [Pt( NNN )CH3][BAr 24], (C10). Complex C10 was isolated and characterized by 1H, 13C{1H}, and 19F{1H} NMR spectroscopy, and the spectra are also indicative

2 of a symmetric species. A similar Pt-CH3 resonance was also observed at 1.21 ppm ( JPt-H = 79 Hz).

Even though Pt-CH3 complexes (C6, C8 and C9) all contain unique secondary sphere N interactions, they contain similar spectroscopic features in the solution phase. When solvated in acetone-d6, a

1 2 slight change in the H NMR resonance of the Pt-CH3 moiety from C6 (1.34 ppm, JPt-H = 82 Hz) to

2 2 C8 (1.42 ppm, JPt-H = 84 Hz) to C9 (1.27 ppm, JPt-H = 79 Hz) was noted. The bond lengths observed in the solid-state structure are also very similar to one another. Notably, the ligand pyrazolate N interactions do not result in a change in the bond lengths of the C-C and C-N bonds within the pyrazolate moiety (e.g. C8: C(1)N(1), 1.357(6) Å and C6: C(1)N(1), 1.358(9) and C9: C(1)N(1),

91 1.357(6) Å). Furthermore, no appreciable difference is observed in the Pt-CH3 bond distance between anionic C6 (2.053(6) Å) and C8 (2.061(3) Å) and cationic C9 (2.064(3) Å).

3.24 Synthesis of *NCN* Ligated Pt-Alkyl Species

H H H H H tBu Addition of 5 equiv. of CH3Li to Pt( NCN ) Cl (C4) and Pt( NCN ) Cl (C5) generated

R’ t [Li]2[Pt(*NCN*) CH3] (C11, R’ = H; C12, R’ = Bu, respectively, Figure 3.06a). C11 and C12

+ were H and O2 sensitive which made their successful reproducible isolation challenging. Isolation of C11 was eventually achieved (45.0 % yield) and crystals suitable for X-ray diffraction were obtained (Figure 3.06a). C11 was not stable in C6D6 for extended time and began decomposing after

12 hours to numerous unidentified species with concomitant methane generation. 1H NMR

II spectroscopy indicated complex C11 is symmetrical in solution. A characteristic Pt -CH3 resonance

2 2 II was observed at 0.09 ppm ( JPt-H of 41 Hz) in dry C6D6. Similar low J has been observed when Pt -

26,27 1 CH3 is trans to aryl moieties. Additionally, bound THF can be observed by H NMR

28 spectroscopy at l.31 ppm and 3.39 ppm in C6D6 (shifted from free THF at 1.47 ppm and 3.47 ppm).

Examination of the solid-state structure of C11 revealed two pyrazolate-Li interactions with THF solvated Li cations at a N-Li distance of 2.00(1) Å, which is similar to the N-Li distance in C6

(2.01(1) Å). While no difference was observed in the Pt-CH3 bond distance of C6 (2.053(6) Å) vs

92 H H t Figure 3.0 6 Reactivity of (a) Pt( NCN )Cl (R = H, C4 and R = Bu, C5) with CH3Li to R t form [Li]2[Pt(*NCN*) CH3] (R = H, C11 and R = Bu, C12) and (b) C4 with C6H5Li to R form[Li]2[Pt(*NCN*) C6H5], C13. Thermal ellipsoid plots (50 % probability) of C11. 4 molecules of THF and H-atoms are omitted for clarity. Selected bond lengths for C11: C(1’)-N(1’) 1.354(7) Å, C(2’)-C(3’) 1.382(7) Å, Li(1’)-N(1’) 2.00(1) Å, Li(1’)-C(21’) 2.32(1) Å, C(1’)C(2’) 1.390(7) Å; N(2’)C(3’), 1.369(6); Pt(1’)C(21’) 2.161(5) and Pt(1’)C(9’) 1.981(5).

C8 (2.061(3) Å), C11 exhibited a longer Pt-CH3 bond distance (2.161(5) Å), possibly due to the dianionic nature of the ligand and/or the stronger trans aryl moiety. Although it could not be isolated, the formation of C12 was confirmed by the observation of methane (which resulted from

93 deprotonation of acidic pyrazole protons with CH3Li reagent) and the appearance of a new methyl

1 2 II signal in the H NMR spectrum at 0.08 ppm ( JPt-H of 43 Hz) in C6D6. This Pt -CH3 resonance is

2 similar to C11 in chemical shift and JPt-H. A possible explanation for the inability to isolate C12 in relation to C11 could be due to the stronger donating ability in the central aryl ring from the additional tBu moiety. This would result in a more electron rich PtII center, promoting protonation reactions at the metal.

The addition of 4.3 equivalents of C6H5Li to a suspension of C5 in THF-d8 yielded complex

tBu [Li]2[Pt(*NCN*) C6H5] (C13, Figure 3.06b) along with two equivalents of benzene and half an equivalent of biphenyl (determined by 1H NMR spectroscopy). C13 was not isolated and subsequent reactions (vide infra) were performed on a crude reaction mixture. The biphenyl appears only after the addition of the second equivalent of phenyl lithium. Biphenyl has been shown to form by C-C cross coupling when aryl lithium and aryl tin reagents are introduced to PtII/PdII complexes.29,30 1H

3 NMR spectroscopy indicated C13 is symmetrical in solution. The Pt-C6H5 peaks at 8.2 ppm ( JPt-H

4 1 = 31 Hz), 7.0 ppm ( JPt-H = 24 Hz) and 7.25 ppm in the H NMR spectrum confirm Pt-C6H5 formation.

+ + II 3.25 Electrophile (H and CH3 ) Addition to Pyrazolate Supported Pt -alkyl Compounds

The reactions of the PtII-alkyl complexes with pyrazolate N in the ligand with electrophiles

+ + (H and/or CH3 ) was probed to determine if electrophile addition was preferred at the ligand or the

II Pt center. We also monitored the reactions to see if direct CH3-R (R = H, C) bond formation occurred. All species investigated have two pyrazolate functionalities built into the ligand framework. Furthermore, the effect of Li coordination to the deprotonated pyrazolate moiety was investigated using complexes containing N-Li interactions. C6 and C7 contain Li coordination to a

94 single pyrazolate N, while C11 and C12 contain Li coordination to both pyrazolate N sites. C8 does not contain any Li coordination. The presence of a Li pyrazolate interaction might inhibit ligand reactivity at this site. The reactions of electrophilic reagents (HX and CH3I) with these complexes was also investigated.

3.25.1 Proton Addition to (*NNN*) ligated PtII-alkyl (C6, C7, C8) Compounds

The reactions of C7 with acids were first investigated. Addition of excess acid (5 equiv. HCl etherate, HBF4 etherate or 2,6-dimethoxypyridinium tetrafluoroborate) to C7 generated monomeric

H H H H [Pt( NNN )(CH2CN)][Cl] (C7a) or [Pt( NNN )(CH2CN)][BF4] (C7b) (68 % and 83 % isolated yield, respectively; Figure 3.07a). Complexes C7a and C7b were symmetrical in solution, evidenced

1 13 1 by H and C{ H} NMR spectroscopy and exhibited similar Pt-CH2CN resonances at 2.87 ppm and

2 2 2.88 ppm (both JPt-H = 101 Hz) in MeOD. The JPt-H coupling of the Pt-CH2CN of C7 (110 Hz) noticeably decreased when the pyrazolate N’s were protonated. Additionally, the Pt-CH2CN appears

II to be stable to 0.5 additional equivalents of acid (per Pt center) as no N bound CH3CN or release of

1 CH3CN was observed by H NMR spectroscopy. A solid-state structure of C7b was obtained (Figure

3.07a) and revealed an H-bonding interaction between pyrazolate N-H and bound CH2CN based on distance (2.09 Å) and N-H-N bond angle (149.5 °). This study indicates addition of Brønsted acid to

C7 reacted exclusively at the ligand to form C7a and C7b. Protonation of the Pt-CH2CN moiety did not occur, even with an additional 0.5 equiv. of acid.

31 Addition of 2,6-dimethoxypyridinium tetrafluoroborate (pKa = 7.6 in CH3CN) to an

1 acetone-d6 solution of C6 and to a CD3CN solution of C8 were monitored by H NMR spectroscopy.

Notably, no Pt-H signals were observed during either of the acid addition experiments and methane elimination was not observed until more than two equivalents of acid had been added to either

95 complex. The results detailed below describe the speciation and overall results of the protonation studies of C6 and C8 (summarized in Figure 3.07b and 1H NMR spectra appear in Figure 3.08).

After addition of one equivalent of acid (either HCl or HBF4 etherate) to C6 or C8, a poorly soluble yellow solid precipitated (labeled [Pt] in Figure 3.07b. and 3.07c) and only very broad features were visible in the 1H NMR spectrum (see 0.8 equiv. and 1.4 equiv. in Figure 3.08a; 1.0 equiv in Figure

3.08b). Attempts to characterize these species independently were unsuccessful due to limited solubility in numerous common laboratory solvents (i.e. pentane, diethyl ether, THF, CH2Cl2, acetone, CH3CN and DMF).

Figure 3.0 7 Protonation of (a) C7 with HCl and HBF4 etherate/2,6 dimethoxy pyridinium H H H H tetrafluoroborate to form [Pt( NNN )(CH2CN)][Cl] (C7a) and [Pt( NNN )(CH2CN)][BF4] (C7b), respectively. (b) Speciation during the protonation of [Li2Cl][Pt(*NNN*)CH3 (C6), H H H H and formation of complexes: [Pt( NNN )CH3][Cl] (C9), [Pt( NNN )Cl][Cl] (C1), and H H [Pt( NNN )Cl][BF4] (C2) in acetone-d6. CDH3 formation most likely proceeds through N-H/D exchange with solvent. No H/D exchange occurs in CD3CN. (c) Speciation during H H H H the protonation of C8: [Pt( NNN )CH3][Cl] (C9), [Pt( NNN )CH3][BF4] (C14), H H H H [Pt( NNN )Cl][Cl] (C1), and [Pt( NNN )NCCD3][(BF4)2] (C3). Thermal ellipsoid plots (50% probability) of C7b and selected bond lengths: Pt(1)-N(3) 2.002(10) Å, Pt(1)-C(18) 2.076(12) Å, H(5’)-N(6) 2.086 Å.

97 With addition of two equivalents of 2,6-dimethoxypyridinium tetrafluoroborate to C6,

H H 1 [Pt( NNN )CH3][Cl] (C9) was observed in the H NMR spectrum (Figure 3.07b, Figure 3.08a at

2.2 equiv., 65 % spectroscopic yield). Addition of HBF4 etherate also resulted in the formation of

C9. To confirm the assignment of C9 as the product in these reactions, an authentic sample of C9

(Section 3.23) was added into the reaction mixtures. Addition of two equivalents of HCl etherate to

C8, the PPN salt analog of C6, was also observed to form C9 (Figure 3.07c, 90% spectroscopic

II II yield). Formation of C9 suggests protonation at the ligand is preferred over Pt /Pt -CH3 protonation.

Complex C9 was stable to thermolysis conditions until 150 °C in a C6D6 suspension, where slow

1 CH4 formation was observed by H NMR spectroscopy. After heating for 15 hours at 180 °C in

C6D6, formation of a single new species was observed after removing the solvent in vacuo and re-

1 dissolving in CD3CN. By H NMR spectroscopy, a symmetrical complex was observed with all

NMR resonances shifted slightly upfield compared to [Pt(HNNNH)Cl][Cl] (C1, ca. 0.2 ppm change for aryl resonances and 0.05 ppm change for the tBu resonance). Examination by ESI-MS revealed a single peak at m/z = 553.1 (corresponding to Pt(HNNNH)Cl+). The similarity in the 1H NMR spectrum to that of C1 suggest that the new compound is Pt(HNNN*)Cl. However, numerous attempts to deprotonate C1 to form Pt(HNNN*)Cl to allow conclusive identification were

t unsuccessful. A variety of bases (KHMDS, LiOH, NEt3, DBU, KO Bu, NaH) were used, but none gave clean reactivity. However, addition of HCl etherate to the thermolysis product reformed C1. If

98 a)

b )

Figure 3.0 8 1H NMR spectra (500 MHz) showing the addition of 2,6- dimethoxypyridinium tetrafluoroborate to a solution of (a) [Li2Cl][Pt(*NNN*)CH3] (C6) in acetone-d6 and (b) [PPN][Pt(*NNN*)CH3] (C8) in CD3CN. In the spectra above, the aryl region is highlighted.

Pt(HNNN*)Cl is the product of the thermolysis reaction, it suggests C-H elimination from

99 cooperation of ligand-based N-H and the Pt-CH3 from C9 is possible, although a very high reaction temperature is required.

An analogous species to C9, C14, was observed in situ when two equivalents of 2,6- dimethoxypyridinium tetrafluoroborate were added to C8 (Figure 3.07c, Figure 3.08b at 2.0 equiv.,

77 % spectroscopic yield). Despite multiple attempts, we were unable to isolate C14 from the addition of acid to C8 or independently through anion exchange of C9 with NaBF4. All resonances

1 1 for C14 are shifted slightly downfield of C9 in the H NMR spectrum in CD3CN. The H NMR signals for complex C14 indicate that it is also symmetrical in solution. Additionally, the Pt-CH3

2 ligand exhibits the same JPt-H of 78 Hz as C9. The broad N-H resonance of C14 is observed downfield at 12.3 ppm in CD3CN, which is similar to that of C9 (12.1 ppm).

Addition of the acid 2,6-dimethoxypyridinium tetrafluoroborate to complex C9 in CD3CN or acetone-d6 yielded C2 (Figure 3.07b) with concomitant methane formation over the course of several days. After protonation, the Cl- counter anion of C9 preferentially coordinates to the unsaturated Pt center to form C2, even in the presence of coordinating solvents, such as acetonitrile. 2,6-lutidinium

31 tetrafluoroborate (pKa = 14.1 in acetonitrile) will also protonate the methyl ligand to form C2 (78

% isolated yield).

Protonation of the methyl ligand of C14 with 2,6- dimethoxypyridinium tetrafluoroborate in

CD3CN formed the dicationic C3-d3 (Figure 3.07c). Confirmation of this species was achieved by

H H comparison with an authentic sample of [Pt( NNN )NCCD3][BF4]2, prepared by stirring non- deuterated C3 in CD3CN and removing the solvent. Here, the acetonitrile solvent traps the unsaturated metal center to form a Pt-solvento complex. Additionally, both C9 and C14 contain protonated ligands with a methyl ligand still intact after two equivalents of added acid. Generation

100 of C9 and C14 in our acid addition studies indicates that the ligand pyrazolate sites are protonated in preference to the metal or metal-methyl bond.

3.25.2 Proton Addition to Pt(*NCN*)CH3 (C11, C12) and Pt(*NCN*)C6H5 (C13)

Addition of H2O (2 equiv) and HCl (1 equiv) at room temperature to C11 and C12, respectively, yielded C4a and C5a (Figure 3.09). C4a and C5a are both symmetrical in solution by

Figure 3.0 9 Speciation during the protonation of [Li2][Pt(*NCN*)R] (R = CH3: [R’ = H t t (C11), R’ = Bu (C12)]; R = C6H5 [R’ = Bu (C13)]) and formation of complexes: R - t H H R’ - t [Li2][Pt(*NCN*) L] (L = Cl : R’ = Bu (C5a), H (C4a)), Pt( NCN ) L (L = Cl [R’ = Bu (C5)]); L = unknown [R’ = H]) in THF-d8. Thermal ellipsoid plots (50 % probability) of C5a. 4 THF molecules and H-atoms omitted for clarity. Selected bond lengths for C5a: Pt(1)-C(1) 1.929(4) Å, Pt(1)-Cl(1) 2.452(2) Å, Li(1)-Cl(1) 2.38(1) Å, Li(1)-N(2) 2.034(9).

101 1 H NMR spectroscopy and release of CH4 was observed when the reaction was monitored in-situ.

The symmetrical nature is further evidenced by a solid-state structure of C5a, which compared to

C11, also contains two N-Li interactions. Similar reactivity to protonation of C4a and C5a was observed when 1 equiv. of HCl was added to in situ generated C13 and formed C5a by 1H NMR

1 spectroscopy. The protonation of C12 with HCl was additionally studied at -73 °C in THF-d8 by H

NMR spectroscopy, where a mixture of products was observed (Figure 3.10). In the 1H NMR spectrum at -73 °C, C12 and C5a are observed, along with an unsymmetrical unknown species and concomitant methane loss. This unknown species contains a resonance centered at 12.02 ppm, likely

Figure 3. 10 VT 1H NMR spectrum (500 MHz) of addition of HCl etherate (1M, 0.016 mmol) to a solution of C12 at -73 °C. corresponding to an N-H of the pyrazole arm. However, there was no observable methyl resonance associated with this intermediate. Upon warming the reaction vessel with 1 equiv. of HCl and C12

102 to room temperature, C5a was the only product observed by 1H NMR spectroscopy. Further protonation of C5a with 2 or 10 equivalents of HCl formed C5. To investigate an acid with a weakly coordinating anion that doesn’t have a driving force for formation of a Pt-X species, 1 equiv. HBF4 etherate was added to a C6D6 solution of C12. Methane was observed, along with a number of unidentifiable resonances in the 1H NMR spectrum. The 1H NMR spectrum converged to a single species when 3 or more equiv. of HBF4 etherate were added, matching those for C4b. These

II II experiments suggest that the Pt /Pt -R of C11, C12 and C13 are too basic compared to the ligand

N, given the propensity to loss of R-H (CH4 or C6H6) upon acid addition. Ligand protonation of C11,

C12 or C13 was not observed and could be due to the strong aryl trans moiety, compared to *NNN* ligated Pt-alkyl species.

II 3.25.3 Reactivity of *NCN* ligated (C11) and *NNN* ligated (C8) Pt -CH3 complexes with CH3I

H 2- Reaction of a C6D6 solution of C11 (Pt(*NCN*) CH3) with 2 equiv. methyl iodide initially revealed a broad symmetrical major and minor Pt complex with Pt-CH3 resonances at 0.72

2 2 ppm ( JPt-H = 43 Hz, 80 % spectroscopic yield) and 0.10 ppm ( JPt-H = 44 Hz, 20 % spectroscopic

1 yield), respectively. No ethane or methane were observed by H NMR spectroscopy. The Pt-CH3 resonances of both complexes integrated to 6H when compared to their respective aryl ligand resonances. Additionally, bound THF can be observed by 1H NMR spectroscopy at l.40 ppm and

3.65 ppm in C6D6 (shifted from uncoordinated THF). As the ligand and two Pt(CH3)2 resonances

IV are broad, there could be possible exchange occurring between different Pt (CH3)2 conformers of

tBu [Li(THF)2]2[Pt(*NCN*) (CH3)2I] (Figure 3.11). Over 16 hours, the major and minor Pt product

103 Figure 3.1 1 Reaction of C11 ([Li(THF)2][Pt(*NCN*)CH3]) with 2 equiv. CH3I to form IV a mixture of two Pt -(CH3)2 complexes, proposed to be tBu [Li(THF)2]2[Pt(*NCN*) (CH3)2I] and a potential 5 coordinate tBu [Li2(THF)4I][Pt(*NCN*) (CH3)2]. These two Pt compounds decompose to form ethane, CH4, CH3D, and unknown Pt complexes.

2 began to diminish with the growth of several new Pt-CH3 resonances at 2.14 ppm ( JPt-H = 69 Hz)

2 and 1.74 ppm ( JPt-H = 77 Hz) and concomitant methane (CH4/CH3D) and ethane formation. Even

1 in the presence of 2 equiv. CH3I, N-methylation was not observed by H NMR spectroscopy, suggesting that reactivity occurs exclusively at PtII. While the exact reason has not yet been determined, it could be due to the inability of ligand N in C11 to act as a nucleophile, having the ligand N-site blocked by Li coordination. However, a more likely explanation is due to the strong trans influence of the aryl moiety, putting more electron density on the PtII center.

104 Figure 3.1 2 Reaction of C6 ([Li2Cl][Pt(*NNN*)CH3]) and C8 ([PPN][Pt(*NNN*)CH3]) IV with 1 equiv. CH3I to form a mixture of two Pt -(CH3) complexes. Addition of KI to the H - [Pt] mixture to form proposed [Pt(*NNN*) (CH3)2I] . Reaction of C8 with 2 equiv. CH3I CH3 H to form [Pt( NNN*) (CH3)2I][I] (C15).

Reaction of a CD2Cl2 solution of C8 with one equiv. of CH3I formed two different

1 symmetrical Pt-(CH3)2 complexes in a 1:4 ratio. By H NMR spectroscopy, the first complex

2 contained Pt-(CH3)2 resonances at 1.93 and 1.03 ppm ( JPt-H = 68, 76 Hz, respectively) and the

2 second contained Pt-(CH3)2 resonances at 1.72 and 0.85 ppm ( JPt-H = 70, 74 Hz, respectively). To attempt to drive the mixture of complexes to a single complex, 1 equiv. KI was added to the CD2Cl2

2 reaction mixture. The added KI perturbed the product distribution in CD2Cl2 to a 60:40 ratio ( JPt-

2 H = 68; 76 Hz and JPt-H = 70; 74 Hz, respectively). Removal of CD2Cl2 solvent and redissolution of the reaction mixture into CD3CN resulted in observation of a single Pt-(CH3)2 species with Pt-

2 1 CH3 resonances at 1.86 and 0.93 ppm ( JPt-H = 69, 75 Hz, respectively) by H NMR spectroscopy.

The change in product distribution might be due to the limited solubility of KI in CD2Cl2. The

105 single Pt-(CH3)2 species is additionally observed when 1 equiv. of CH3I and 1 equiv. of KI is added to a CD3CN solution of C8. Examination of the reaction mixture by ESI-MS in CH3CN revealed a

- single peak at m/z = 673.2 (corresponding to [Pt(*NNN*)(CH3)2I] ). No evidence of N-methylation was observed by 1H NMR spectroscopy. Additionally, whether the counter cation to the likely

- + + product ([Pt(*NNN*)(CH3)2I] ) is PPN or K is unknown. This could be probed by addition of crown ether, which would shift the resonance of unbound crown ether if K+ was present. The proposed reactivity and speciation is summarized in Figure 3.12. Addition of excess CH3I (> 2 equiv.) to a solution of C6 or C8 in THF-d8 or acetone-d6, respectively, formed

CH3 1 Pt( NNN*)(CH3)2I (C15) as the main product by H NMR spectroscopy (Figure 3.12) and this product was isolated. Examination of the 1H NMR spectrum revealed an unsymmetrical ligand

2 2 environment, two Pt-(CH3) resonances at 2.37 ppm ( JPt-H = 69 Hz) and 1.19 ppm ( JPt-H = 70 Hz) and a ligand N-CH3 resonance at 4.23 ppm. Additionally, ESI-MS revealed the main Pt product

CH3 H + contained a m/z = 688.2 (corresponding to [Pt( NNN )(CH3)2I] ). It appears methyl iodide reacts

II first with the Pt center to form a mixture of two different Pt-(CH3)2 complexes, which can further

- be pushed to one Pt-(CH3)2 species, likely [Pt(*NNN*)(CH3)2I] by KI addition. Addition of 2 total

II IV equiv. of CH3I to C8 not only oxidizes Pt and forms a new Pt -(CH3) bond, but methylates the ligand N to form C15 as the main Pt product.

3.3 Conclusion

A series of PtII-Cl and PtII-alkyl complexes supported by 2,6-bis(5-tert-butyl-1H-pyrazol-3- yl)pyridine (HNNNH) and 5-tert-butyl-1,3-bis(pyrazol-3-yl)benzene (HNCNH)R (R = H, tBu) ligands were synthesized and characterized. Once formed, the reactivity of PtII-alkyl complexes with electrophilic reagents (HX and CH3I) was explored and is summarized below.

106 R t Addition of 1 equiv. of acid to [Li(THF)2]2[Pt(*NCN*) CH3] (R = H, C11; R = Bu, C12)

R and [Li(THF)2]2[Pt(*NCN*) C6H5] (C13) generated R-H (R = CH3 or C6H5) and the corresponding

II R t Pt complex, [Li(THF)2]2[Pt(*NCN*) Cl] (R = H, C4a; R = Bu, C5a), even at reduced temperatures. However, acid addition experiments to [Li2Cl][Pt(*NNN*)CH3] (C6),

[Li(THF)]2[Pt(*NNN*)CH2CN]2 (C7), and [PPN][Pt(*NNN*)CH3] (C8) revealed ligand protonation in preference to Pt/Pt-CH3 protonation. NMR spectroscopy and solid-state studies revealed C6 and C8 can both uptake two equivalents of acid without methane liberation. Addition of >2 equivalents of acid to C6 and C8 liberates methane. In all, tridentate pyrazolate ligated PtII-

R systems can store protons before R-H elimination if there is a weak trans donor to the PtII-R bond (pyridine vs phenyl) and/or if the Pt complex is not dianionic (C11, C12 and C13 vs C6 and

C8).

H IV Reaction of 2 equiv. of CH3I with C11 ([Li(THF)2]2[Pt(*NCN*) (CH3)]) formed two Pt -

tBu CH3 species, whose identity is likely [Li(THF)2]2[Pt(*NCN*) (CH3)2I] or a 5-coordinate analog,

1 with no evidence of N-methylation products formed, as evidenced by H NMR spectroscopy.

IV Addition of 1 equivalent of CH3I with [PPN][Pt(*NNN*)CH3] (C8) also reacted to form two Pt -

IV (CH3)2 products. Addition of KI to this mixture resulted in a single Pt product, likely

- [Pt(*NNN*)(CH3)2(I)] . Addition of two equiv. of CH3I to [Li2Cl][Pt(*NNN*)CH3] (C6) or C8

CH3 generates the oxidized N-methylation product Pt( NNN*)(CH3)2I (C15). While reactivity of 2

equiv. CH3I to C11 did not appear to form any N-CH3 product, N-alkylation was observed when

+ + CH3I added to C6 or C8. Similar to the H addition experiments, ligand reactivity with CH3 was

II II observed for NNN ligated Pt -CH3 species as opposed to an NCN ligated Pt -CH3 complex, where no ligand N-alkylation was observed by 1H NMR spectroscopy.

107 3.4 Experimental and NMR Data

3.41 General Experimental

All manipulations were carried out under nitrogen atmosphere using standard Schlenk and glovebox techniques unless otherwise noted. Deuterated solvents were purchased from Cambridge

Isotope Laboratories. Dry tetrahydrofuran, benzene, pentane, methylene chloride, acetone, acetonitrile, and diethyl ether were obtained by means of a Grubbs-type solvent purification

32 system. THF-d8 and C6D6 were dried over sodium/benzophenone ketyl and were vacuum transferred prior to use. Acetone-d6 and CD3CN were dried over activated 3 Å molecular sieves.

CD2Cl2 was dried over calcium hydride and vacuum transferred prior to use. PtCl2(S(CH3)2)2,

Pt(C6H5)2(S(CH3)2 and [Pt(CH3)2(µ-S(CH3)2)]2 were synthesized following literature

33,34 preparations. K2PtCl4 was purchased from Pressure Chemicals. All NMR spectra were obtained on a Bruker Avance 500, Bruker Avance 400 or Bruker Avance 300 MHz instrument.

The spectra were recorded at 300 K. Chemical shifts are reported in units of parts per million

(ppm) downfield of TMS and referenced against residual protonated solvent resonances (1H) and

13 31 1 characteristic solvent resonances ( C). P{ H} NMR spectra were referenced externally to H3PO4

(85%, 0 ppm) and 2H NMR spectra were referenced to the deuterium resonance of extra added

19 1 CD3CN (δ 1.94). F{ H} NMR spectra were referenced externally to C6H5F (-113.15

7 1 ppm). Li{ H} NMR spectra were referenced externally to LiCl in D2O (0.0 ppm). NMR tubes fitted with a J-Young style Teflon valve were used to obtain inert atmosphere NMR data. The C,

N, H elemental analyses were carried out at the CENTC Elemental Analysis Facility at the

University of Rochester. Accurate mass measurement analyses were conducted on either a Waters

GCT Premier, time-of-flight, GCMS with electron ionization (EI), or an LCT Premier XE, time- of-flight, LCMS with electrospray ionization (ESI). Samples were taken up in a suitable solvent

108 for analysis. The signals were mass measured against an internal lock mass reference of perfluorotributylamine (PFTBA) for EI-GCMS, and leucine enkephalin for ESI-LCMS. Waters software calibrates the instruments, and reports measurements, by use of neutral atomic masses.

The mass of the electron is not included. Nominal mass accuracy ESI-MS data were obtained by use of a Waters Acquity UPLC system equipped with a Waters TUV detector (254 nm) and a

Waters SQD single quadrupole mass analyzer with electrospray ionization.

3.42 Synthesis, Characterization and Spectroscopic Data

[Pt(HNNNH)Cl][Cl] (C1)

A 100 mL Schlenk flask was charged with 282.5 mg (0.873 mmol) of HNNNH, 339.1 mg (0.869 mmol) Pt(S(CH3)2)Cl2 and 15 mL methanol in air. The solution was sparged with N2 and heated at reflux for 2 hours to yield a yellow solution. The solution was cooled to room temperature and then concentrated under reduced pressure. Diethyl ether (20 mL) was added, precipitating a yellow solid. The solid was collected, re-dissolved in methanol and concentrated under reduced pressure. Diethyl ether was again added, and the suspension was filtered via a fritted funnel. The yellow solid was collected and dried under reduced pressure (437.1 mg, 84.9 %). 1H

3 3 NMR (CH3OD, 500 MHz): δ 8.27 (1H, t, pyr, JH-H = 8.0 Hz), 7.96 (2H, s, pyr, JH-H = 8.0 Hz),

t 13 1 7.09 (2H, s, pyz), 1.46 (18H, s, Bu). C{ H} (CH3OD, 126 MHz): δ 162.83 (s), 158.16 (s), 154.78

109 (s), 146.30 (s), 123.62 (s), 107.59 (s), 35.96 (s), 32.65 (s). Elemental Analysis: Anal. For

C17H25ClN4Pt•2H2O: Calc: C, 36.49; H, 4.67; N, 11.20. Found: C, 36.68; H, 4.55; N, 11.23.

1 Figure 3.13. H NMR spectrum (500 MHz) of C1 in MeOD

13 1 Figure 3.14. C{ H} NMR spectrum (126 MHz) of C1 in MeOD

110 H H [Pt( NNN )Cl][BF4] (C2)

A 20 mL scintillation vial was charged with 23.5 mg (0.0399 mmol) C1, a Teflon stir bar, 4.4 mg (0.399 mmol) sodium tetrafluoroborate, and acetonitrile (2 mL) in air. The suspension was stirred vigorously for 1 hour. The resulting suspension was filtered by fritted funnel and extracted with CH2Cl2 (2 x 2 mL). The volatiles were removed from the resulting filtrate by rotary evaporation to yield a yellow solid. The solid was re-dissolved in methanol (10 mL) and concentrated to (~3 mL). Pentane (20 mL) was added and the resulting suspension was filtered via

1 fritted funnel. The solid was collected and dried (21.0 mg, 82.1 %). H NMR (CD3CN, 500 MHz):

3 3 δ 12.07 (2H, br, N-H), 8.21 (1H, t, pyr, JH-H = 8.0 Hz), 7.81 (2H, d, pyr, JH-H = 8.0 Hz), 6.99 (2H,

t 1 3 s, pyz), 1.43 (18H, s, Bu), H NMR (CD3OD, 400 MHz): 8.22 (1H, t, pyr, JH-H = 7.8 Hz), 7.89

3 t 13 1 (2H, d, pyr, JH-H = 7.8 Hz), 7.03 (2H, s, pyz), 1.45 (18H, s, Bu) C{ H} (CD3CN, 126 MHz): δ

159.98 (s), 155.33 (s), 151.67 (s), 143.63 (s), 121.06 (s), 105.04 (s), 33.09 (s), 29.86 (s). 19F NMR

(CD3CN, 377 MHz): δ -154.47 (s), -154.52 (s). Elemental Analysis: Anal. For

C19H25BClF4N5Pt•2H2O: Calc: C, 33.72; H, 4.32; N, 10.35. Found: C, 33.00; H, 3.82; N, 10.46.

111

1 Figure 3.15. H NMR spectrum (400 MHz) of C2 in CD3CN

13 1 Figure 3.16. C{ H} NMR spectrum (126 MHz) of C2 in CD3CN

112

19 1 Figure 3.17. F{ H} NMR spectrum (376 MHz) of C2 in CD3CN

H H [Pt( NNN )NCCH3][(BF4)2] (C3)

A 20 mL scintillation vial was charged with 31.3 mg (0.0533 mmol) C1, a Teflon stir bar, 20.7 mg (0.107 mmol) silver tetrafluoroborate, and acetonitrile (3 mL). The suspension was stirred vigorously for 1 hour in the absence of light. The resulting suspension was filtered through a PTFE syringe filter. The filtrate was concentrated under reduced pressure (~ 1 mL) and diethyl ether (5 mL) was added to precipitate a light yellow solid. The mother liquor was decanted and the solid was washed with ether (2 x 5 mL) and dried under reduced pressure (27.2 mg, 66.5

1 3 %). H NMR (CD2Cl2, 500 MHz): δ 12.92 (2H, s, N-H), 8.23 (1H, t, pyr, JH-H = 8.0 Hz), 7.72 (2H,

3 t 1 d, pyr, JH-H = 8.0 Hz), 6.75 (2H, s, pyz), 2.94 (3H, s, NCCH3) 1.43 (18H, s, Bu), H NMR

3 3 (CD3CN, 500 MHz): 12.13 (2H, s, N-H), 8.29 (1H, t, pyr, JH-H = 8.0 Hz), 7.88 (2H, d, pyr, JH-H =

t 13 1 8.0 Hz), 7.05 (2H, s, pyz), 1.45 (18H, s, Bu), C{ H} (CD3CN, 126 MHz): δ 160.91 (s), 155.09

113 19 (s), 152.25 (s), 146.05 (s), 121.66 (s), 105.65 (s), 33.13 (s), 29.81 (s) F NMR (CD3CN, 377 MHz):

δ -150.89 (s), -150.94 (s). Elemental Analysis: Anal. For C21H28B2F8N6Pt: Calc: C, 34.40; H, 3.85;

N, 11.46. Found: C, 34.21; H, 3.52; N, 11.22.

1 Figure 3.18. H NMR spectrum (500 MHz) of C3 in benchtop CD2Cl2

1 Figure 3.19. H NMR spectrum (500 MHz) of C3 in CD3CN

114

13 1 Figure 3.20. C{ H} NMR spectrum (126 MHz) of C3 in CD3CN

Figure 3.21. 19F{1H} NMR spectrum (376 MHz) of C3 in MeOD

Pt(HNCNH)HCl (C4)

In a 50 mL round bottomed flask, 47.2 mg (0.202 mmol) of (HNCNH)H and 51.8 mg

(0.125 mmol) of potassium tetrachloroplatinate were added. Glacial acetic acid (25 mL) was added

115 and the resulting suspension was heated to reflux (ca. 118 °C). Water (1.5 mL) was slowly added through the condenser to yield a yellow solution. After 48 hours, the solution became a yellow suspension and the reaction was cooled. To the suspension was added, H2O (50 mL). The solid was collected via filtration using a fritted glass funnel to yield an oily solid. The oily solid was washed with H2O (2 x 20 mL). The solid was collected by extraction with MeOH and filtration through the frit. The solvent of the resulting filtrate was removed in vacuo to yield a yellow solid

(48.2 mg, 87 %). Irreproducibility plagued this reaction; repeating this reaction for further characterization to obtain the same main product could not be achieved. Attempts were made to vary the reagent stoichiometry, reagent conditions (solvent, O2 vs N2 and without ambient light)

II 1 3 and Pt starting materials. H NMR (CDCl3, 500MHz): δ 10.72 (2H, s, NH), 7.20 (2H, d, pyr, JAr-

3 t H = 7.1 Hz), 7.12 (1H, d, pyr, JAr-H = 7.1 Hz), 6.36 (2H, s, pyz), 1.40 (18H, s, Bu).

1 Figure 3.22 H NMR spectrum (500 MHz) of C4 in C6D6.

116 tBu [Li(THF)2]2[Pt(*NCN*) Cl] (C4A)

A 500 mL Schlenk tube was charged with C5 (68.2 mg, 0.124 mmol), THF (70 mL) and a stir bar.

The resulting suspension was put in a freezer (-35 °C) for 45 mins. A 1.6 M diethyl ether solution of CH3Li (386 µL, 0.618 mmol) was added and the resulting mixture was allowed to warm to room temperature and stir for 2 hours. To the mixture, H2O (4.5 µL, 0.25 mmol) was added, allowed to stir for 30 mins. The reaction mixture was filtered through a fritted funnel. The solvent was removed from the filtrate under reduced pressure and an NMR was taken of the resulting solid. 1H

3 NMR (CD3CN, 500MHz): δ 6.86-6.91 (1H, m, pyr), 6.83 (2H, d, pyr, JAr-H = 7.1 Hz), 6.12 (2H, s, pyz), 1.34 (18H, s, tBu).

1 Figure 3.23 H NMR spectrum (500 MHz) of C4A in CD3CN.

117 Pt(HNCNH)tBuCl (C5)

In a 250 mL round bottom flask, 395 mg (1.06 mmol) of (HNCNH)tBu and 439 mg (1.06 mmol) of potassium tetrachloroplatinate were added. Glacial acetic acid (100 mL) was added and the resulting suspension was heated to reflux (ca. 118 °C). Water (27 mL) was slowly added through the condenser to yield a yellow solution. After 48 hours, the solution became a yellow suspension and the reaction was cooled. To the suspension was added, H2O (40 mL). The solid was collected via filtration using a fritted glass funnel to yield a green oily solid. THF (50 mL) was added to the oily solid to form a yellow green suspension. After an hour, solid black particles were observed, and the suspension was filtered via fritted glass funnel. The solvent was

1 removed in vacuo overnight to yield a yellow solid. (561 mg, 88 %). H NMR (CDCl3, 300MHz):

4 t δ 10.60 (2H, s, NH), 7.29 (2H, d, pyr, JAr-H = 4.5 Hz), 6.40 (2H, m, pyz), 1.42 (18H, s, Bu), 1.39

(9H, s, tBu).

118

1 Figure 3.24. H NMR spectrum (300 MHz) of C5 in CDCl3

tBu [Li(THF)2]2[Pt(*NCN*) Cl] (C5A)

A 20 mL scintillation vial was charged with 2 mL of THF, a stir bar, and C5 (33.3 mg, 0.0548 mmol). The resulting suspension was cooled to -30 °C, and 69 µL of a 1.6 M (0.11 mmol) diethyl ether solution of CH3Li was added. The THF/diethyl ether solution was put in a -35 °C freezer and crystals formed. A small amount of the crystals was collected and an 1H NMR spectrum in THF-

1 d8 was recorded below. H NMR (THF-d8, 500MHz): δ 6.87 (2H, s, pyr), 6.04 (2H, s, pyz), 6.12

(2H, s, Ar-H), 1.33 (9H, s, tBu), 1.32 (18H, s, tBu).

119

1 Figure 3.25. H NMR spectrum (300 MHz) of C5A in THF-d8

[Li2Cl(THF)4][Pt(*NNN*)CH3] (C6)

A 500 mL Schlenk tube was charged with 179 mg (0.325 mmol) of C1 and 200 mL THF. While vigorously stirring, 1.5 mL (1.95 mmol) of CH3Li (1.3 M in diethyl ether) was added via syringe under a nitrogen atmosphere and stirred for 2 hours. Water (15.6 µL, 0.98 mmol) was added via microliter syringe and stirred for 5 minutes. The solution was concentrated to 5 mL, and pentane was added (50 mL) to yield a dark solid, which was filtered and discarded. The filtrate

1 was dried in vacuo to yield a yellow solid. (173.7 mg, 57.2 %). H NMR (THF-d8, 500 MHz): δ

3 3 7.61 (1H, t, pyr, JH-H = 7.8 Hz), 7.10 (2H, d, pyr, JH-H = 7.8 Hz), 6.38 (2H, d, pyz), 1.32 (1H, s, t 2 13 1 Bu), 1.11 (3H, s, Pt-CH3, JPt-H = 78 Hz). C{ H} NMR (THF-d8, 126 MHz): δ 160.65 (s), 153.52

(s), 153.21 (s), 138.56 (s), 112.06 (s), 100.39 (s), 32.79 (s), 31.39 (s), -24.03 (s). 7Li{1H} NMR

120 H H + (acetone-d6, 155.5 MHz): δ -1.40 (s). ESI-MS: [Pt( NNN )CH3] Theoretical Mass, m/z =

532.1972; Observed Mass, m/z = 532.1952.

1 Figure 3.26. H NMR spectrum (500 MHz) of C6 in THF-d8.

13 1 Figure 3.27. C{ H} NMR spectrum (126 MHz) of C6 in THF-d8

121

7 1 Figure 3.28. Li{ H} NMR spectrum (376 MHz) of C6 in Acetone-d6

[Li(THF)]2[Pt(*NNN)CH2CN]2 (C7)

A 200 mL round bottom was charged with 123.5 mg (0.210 mmol) C1, 100 mL of THF, and a stir bar to yield a yellow suspension. While vigorously stirring, 786 µL of a 1.6 M diethyl ether solution of CH3Li (0.904 mmol) was added and the resulting blue solution was stirred for 2.2 hours. Water (11 µL) was added and effervescence was observed. The solution was concentrated to 10 mL and 100 mL of pentane was added to yield a yellow suspension. The suspension was filtered through a fritted funnel and the solid discarded. The solvent was removed from the filtrate under reduced pressure. The yellow solid was collected and further dried under reduced pressure

1 (97.4 mg, 66 % yield). Note - the source of the CH2CN moiety in the product is unknown. H NMR

3 3 (Acetone-d6, 500 MHz): δ 7.70 (2H, t, pyr, JH-H = 7.8 Hz), 7.18 (4H, d, pyr, JH-H = 7.8 Hz), 6.43

2 t 13 1 (4H, d, pyz), 2.50 (2H, s, Pt-CH3, JPt-H = 110 Hz) 1.31 (1H, s, Bu). C{ H} NMR (THF-d8, 126

122 MHz): δ 160.94 (s), 154.04 (s), 152.75 (s), 140.09 (s), 132.54 (s), 111.97 (s), 100.91 (s), 68.11 (s),

2 7 1 32.98 (s), 31.60 (s), 26.21 (s), -23.06 (s, JPt-H = 723 Hz). Li{ H} NMR (acetone-d6, 155.5 MHz):

δ 0.96 (s).

1 Figure 3.29. H NMR spectrum (500 MHz) of C7 in Acetone-d6

13 1 Figure 3.30. C{ H} NMR spectrum (126 MHz) of C7 in Acetone-d6

123

7 1 Figure 3.31 Li{ H} NMR spectrum (156 MHz) of C7 in CD3CN

[PPN][Pt(*NNN*)CH3] (C8)

A 25 mL Schlenk flask was charged with a Teflon stir bar, 6 (42.6 mg, 0.0792 mmol), [PPN][Cl] (43.0 mg, 0.0752 mmol) and 10 mL of THF. The suspension was vigorously stirred for 1 hour to precipitate a white solid. The solid was separated by transferring the solvent using a cannula with a filter tip. The solvent was then removed under reduced pressure from the filtrate to yield a solid, which was extracted with 3 mL of CH2Cl2. The resulting solution was dried 1 in vacuo to yield an orange solid (63.0 mg, 74.3 %). H NMR (CD3CN, 400 MHz): δ 7.63 (1H, t, 3 3 pyr, JH-H = 7.9 Hz), 7.58 (36H, multiplet, PPN), 7.10 (2H, d, pyr, JH-H = 7.9 Hz), 6.42 (2H, s, t 2 31 1 pyz), 1.31 (18H, s, Bu), 1.11 (3H, s, Pt-CH3, JPt-H = 82 Hz). P{ H} (CD3CN, 162 Hz): δ 22.00. 13 1 C{ H} NMR (CD3CN, 126 MHz): δ 161.32 (s), 153.53 (s), 153.03 (s), 139.01 (s), 133.17 (m, 1 3 PPN), 130.30 (m, PPN), 128.95 (dd, JC-P = 107.8 Hz, JC-P = 1.8 Hz, PPN), 112.15 (s), 100.52 (s), 32.87 (s), 31.50 (s), -18.28 (s) Attempts to characterize the material by Elemental Analysis were H H + not successful. ESI-MS: Theoretical Mass [Pt( NNN )CH3] m/z = 532.1972; Observed Mass, m/z = 532.1967.

124

1 Figure 3.32. H NMR spectrum (400 MHz) of C8 in CD3CN

13 1 Figure 3.33. C{ H} NMR spectrum (125 MHz) of C8 in CD3CN. Low S/N sensitivity of the Pt- 13 CH3 resonance at -18.28 ppm. Confirmed C signal in HSQC of C8.

125

Figure 3.34. HSQC NMR spectrum (500, 126 MHz) of C8 in CD3CN. Pt-CH3 resonance circled for clarity.

31 1 Figure 3.35. P{ H} NMR spectrum (162 MHz) of C8 in CD3CN

H H [Pt( NNN )CH3][Cl] (C9)

126 A 20 mL scintillation vial was charged with 137 mg (0.233 mmol) C1, a Teflon stir bar, and benzene (10 mL). While vigorously stirring, a 2.9 M CH3MgCl (0.337 mL, 0.977 mmol) was added via syringe and stirred for 20 mins. A water (17 µL) and THF (2 mL) solution was added and the reaction mixture was let stir for 20 minutes. The resulting suspension was filtered by fritted funnel and the orange and white solid was dried (155.2 mg). Note: THF 1 resonances in the H NMR spectrum likely due to MgX2 impurities, as a less than 100 % yield was 1 not attainable, even after rigorous drying. H NMR (Acetone-d6, 400 MHz): δ 13.28 (2H, br, N- 3 3 H), 8.34 (1H, t, pyr, JH-H = 8.0 Hz), 8.11 (2H, d, pyr, JH-H = 8.0 Hz), 7.20 (2H, s, pyz), 1.47 (18H, t 2 s, Bu), 1.24 (3H, s, Pt-CH3, JPt-H = 79 Hz).

1 Figure 3.36. H NMR spectrum (400 MHz) of C9 in Acetone-d6.

127 H H 24 [Pt( NNN )CH3][BArF ] (C10)

A 20 mL scintillation vial was charged with 46.0 mg (0.078 mmol) C1, a Teflon stir bar, and benzene (3 mL). While vigorously stirring, 0.117 mL (0.328 mmol) of CH3MgCl (2.9 M in THF) was added via syringe and stirred for 20 mins. A solution of water (5 µL, 0.224 mmol) and THF (1 mL) was added and the reaction mixture was let stir for 20 minutes. The resulting suspension was filtered by a fritted funnel and the mixture of orange and white solid was collected into a 20 F mL scintillation vial (35.5 mg). The vial was additionally charged with NaBAr 24 (49.8 mg, 0.0562 mmol), acetone (3 mL) and a Teflon stir bar and stirred for 1.5 hours to form a suspension. The mixture was reduced by half under vacuum and 10 mL of diethyl ether was added. The resulting suspension was filtered by PTFE syringe filter. The filtrate was concentrated under reduced pressure to 1 mL, and 10 mL of pentane was added to form a suspension. The mother liquor was decanted, and the resulting orange oil was pumped to dryness to form an orange solid (62.7 mg, 1 3 56.7 % yield). H NMR (Acetone-d6, 500 MHz): δ 13.14 (2H, s, N-H), 8.38 (1H, t, pyr, JH-H = 8.0 3 F F Hz), 8.17 (2H, d, pyr, JH-H = 8.0 Hz), 7.78 (8H, s, BAr 24), 7.78 (4H, s, BAr 24), 7.24 (2H, s, pyz), t 2 13 1 1.46 (18H, s, Bu), 1.21 (3H, s, Pt-CH3, JPt-H = 79 Hz). C{ H} (Acetone-d6, 126 MHz): δ 162.76 1 3 (q, JBC = 50.0 Hz), 159.07 (s), 157.2 (s), 149.94 (s), 142.47 (s), 135.70 (s), 130.18 (q, JCF = 32.6 1 Hz), 125.51 (q, JCF = 271.9 Hz), 120.75, 118.6 (s), 104.87 (s), 32.99 (s), 30.75 (s), -22.69 (s). 19 1 H H + F{ H} NMR (CD2Cl2, 377 MHz): δ -62.85 (s). ESI-MS: [Pt( NNN )CH3] Theoretical Mass, m/z = 532.1972; Observed Mass, m/z = 532.2000.

128

1 Figure 3.37. H NMR spectrum (400 MHz) of C10 in Acetone-d6.

13 1 Figure 3.38. C{ H} NMR spectrum (126 MHz) of C10 in Acetone-d6.

129

19 1 Figure 3.39. F{ H} NMR spectrum (400 MHz) of C10 in Acetone-d6.

H [Li(THF)2]2[Pt(*NCN*) (CH3)] (C11)

A 50 mL Schlenk flask was charged with C4 (37.7 mg, 0.0683 mmol), 20 mL

THF, and a stir bar. The solution was cooled to -35 °C and 5 equiv. of 1.5 M CH3Li (228 µL) in diethyl ether was added while vigorously stirring. After 45 mins, the volume was reduced to 5 mL in-vacuo. 20 mL of pentane was added, causing yellow solid to fall out of solution. After 45 mins, the suspension was carefully decanted and then redissolved in THF. The solution was concentrated in-vacuo and 10 mL pentane was added, precipitating out a yellow solid. The suspension was again decanted, and the yellow solid was extracted with benzene and further dried (25.5 mg, 0.031

1 3 mmol). H NMR (THF-d8, 500 MHz): δ 7.25 (2H, d, pyr, JH-H = 7.2 Hz), 7.16-7.22 (1H, m, pyr),

6.32 (2H, d, pyz), 3.36 (16H, m, THF), 1.48 (18H, s, tBu), 1.28 (16H, m, THF), 0.06 (3H, s, Pt-

2 CH3, JPt-H = 41 Hz).

130

1 Figure 3.40 H NMR spectrum (500 MHz) of C11 in C6D6

tBu [Li(THF)2]2[Pt(*NCN*) (CH3)] (C12)

A J. Young tube was charged with 10 mg (0.016 mmol) of C5 under nitrogen. THF-d8 (0.4 mL) was added via vacuum transfer to afford a yellow solution. The solution was cooled to -35 °C and then 34 µL (0.057 mmol) of methyllithium (1.6 M) in diethyl ether was added. The solution became a lighter yellow shade and C12 was formed cleanly. 1H NMR (THF-

t t d8, 500MHz: δ 6.94 (2H, s, pyr), 6.01 (2H, s, pyz), 1.33 (9H, s, Bu), 1.32 (18H, s, Bu), 0.07 (3H,

2 s, Pt-CH3, JPt-H = 44 Hz).

131

1 Figure 3.41 H NMR spectrum (500 MHz) of C12 in-situ in THF-d8. Diethyl ether was present from HCl etherate addition. Hexamethylbenzene (HMB) is present for internal reference.

tBu [Li]2[Pt(*NCN*) (C6H5)] (C13)

A J. Young tube was charged with 5.3 mg (0.0090 mmol) of 5 under nitrogen. THF-d8 was added via vacuum transfer to yield a yellow solution. The solution was cooled to -35 °C and then 19.2 µL (0.035 mmol) of phenyllithium (1.6 M) in n-butyl ether was

1 added. The solution became a lighter yellow shade. H NMR (THF-d8, 500MHz): δ 8.15 (2H, m,

3 4 Ar-H, JPt-H = 31 Hz), 7.23 (1H, m, pyr JPt-H = 24 Hz), 7.00 (2H, m, Ar-H), 6.98 (2H, s, pyr), 6.04

(2H, s, pyz), 1.36 (9H, s, tBu), 1.26 (18H, s, tBu).

132

1 Figure 3.42. H NMR spectrum (300 MHz) of C13 in-situ in THF-d8. Formation of biphenyl and benzene shown in the magnified aryl region. Hexamethylbenzene (HMB) used as an internal standard.

CH3 Pt( NNN*)(CH3)2(I) (C15)

A J. Young tube was charged with 4.4 mg (0.0041 mmol) of C8 under nitrogen.

Acetone-d6 (0.4 mL) was added under N2 to yield a yellow solution. To the solution, 2.5 equivalent of CH3I (1.0 µL, 0.016 mmol) was added. The reaction was vigorously shaken and allowed to react for 6 hours. A 1H NMR spectrum was obtained and is shown below. The volatiles were then removed under reduced pressure. The resulting yellow solid was washed with pentane (1 x 1 mL) and collected with diethyl ether (1 x 1 mL). The ethereal solvent was removed by bubbling air through the solution over 30 mins to yield a yellow solid. The following 1H NMR spectrum of the

133 1 3 yellow solid was recorded in CD3CN. H NMR (acetone-d6, 500MHz): δ 8.14 (1H, t, pyr, JH-H =

3 3 7.9 Hz), 7.95 (1H, d, pyr, JH-H = 7.9 Hz), 7.83 (1H, d, pyr, JH-H = 7.9 Hz), 7.19 (1H, s, pyz), 6.74

2 t t (1H, s, pyz), 4.22 (3H, s, N-CH3), 2.37 (3H, s, JPt-H = 69 Hz), 1.52 (9H, s, Bu), 1.32 (9H, s, Bu),

2 1 3 (3H, s, JPt-H = 70 Hz). H NMR (CD3CN, 500MHz): δ 8.07 (1H, t, pyr JH-H = 8.0 Hz), 7.76 (1H,

3 4 3 4 dd, pyr JH-H = 8.0 Hz, JH-H = 0.6 Hz), 7.73 (1H, dd, pyr, JH-H = 8.0 Hz, JH-H = 0.6 Hz), 7.01 (1H,

2 t s, pyz), 6.77 (1H, s, pyz), 4.08 (3H, s, N-CH3), 2.28 (3H, s, JPt-H = 68 Hz), 1.47 (9H, s, Bu), 1.33

t 2 (9H, s, Bu), 1.24 (3H, s, JPt-H = 70 Hz).

1 Figure 3.43. H NMR spectrum (500 MHz) of C15 in-situ in acetone-d6.

134

1 Figure 3.44. H NMR spectrum (500 MHz) of C15 in benchtop CD3CN

NMR Data for the portion-wise addition of acid

Acid addition to C6 and C8

A J. Young NMR tube was charged with C6 or C8 and an internal standard (hexamethyl benzene) under nitrogen. Acetone-d6 or CD3CN (0.4 mL for both) were vacuum transferred to the NMR tube. An initial NMR spectrum was recorded, and then 2,6-dimethoxypyridinium tetrfluoroborate acid was added (amounts shown in Figure 3.08a and 3.08b) in a nitrogen filled glovebox. After each acid addition, the NMR tube was sealed, agitated and an NMR spectrum was recorded (Figure 3.08a and 3.08b).

135 3.43 X-ray Crystallography General Information

H H Complexes C2, C3, C4, (Pt4(*NCN*)( NCN )3), C6, C8, C9, and C11 : X-ray intensity data were collected on a Bruker APEXIII D8QUEST35 CMOS area detector, both employing graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) at 100(1) K. Preliminary in indexing was performed from a series of twenty-four 0.5° rotation frames with exposures of 10 seconds. Rotation frames were integrated using SAINT36, producing a listing of unaveraged F2 and σ(F2) values. The intensity data were corrected for Lorentz and polarization effects and for absorption using

SADABS.37 The structure was solved by direct methods – ShelXT.38 Refinement was by full- matrix least squares based on F2 using SHELXL-2018.39 All reflections were used during

2 2 2 refinement. The weighting scheme used was w=1/[σ (Fo )+ (0.0612P) + 1.0285P] where P =

2 2 (Fo + 2Fc )/3. Non-hydrogen atoms were refined anisotropically and hydrogen atoms were refined using a riding model.

Complexes C5a: X-ray intensity data were collected at -173 °C on a Bruker APEX II single crystal

X-ray diffractometer, Mo-radiation. The data was integrated and scaled using SAINT, SADABS within the APEX2 software package by Bruker.40 Solution by direct methods (SHELXS,

SIR97)41,42 produced a complete heavy atom phasing model consistent with the proposed structure.

The structure was completed by difference Fourier synthesis with SHELXL97.43,44 Scattering factors are from Waasmair and Kirfel.45 Hydrogen atoms were placed in geometrically idealised positions and constrained to ride on their parent atoms with C---H distances in the range 0.95-1.00

136 Complex C2 C3 C4

Emperical Formula C21.5H30.625BClF4N5.625O1.375Pt C21H28B2F8N6Pt C20H27ClN4OPt Formula weight 707.24 733.2 569.99 Temperature (K) 100 100 100 Wavelength (Å) 0.71073 0.71073 0.71073 Crystal System orthorhombic triclinic trigonal

Space group Pca21 PT P3121 a =29.3015(12), b = 13.7435(6), c a = 10.8637(4), b =11.2865(4), c Unit cell axes (Å) =26.6867(12) =11.9333(4) a = 11.0452(4), c = 14.8251(7) α = 65.3770(10), β = 83.4580(10), γ = Unit cell angles (°) orthorhombic 76.1960(10) trigonal Volume (Å3) 10746.9(8) 1291.54(8) 1566.30(14) Z 16 2 3 Demsity (mg/m3), calc. 1.748 1.885 1.813 Absorption coeff. (mm-1) 5.376 5.513 6.864 F(000) 5536 712 834 Crystal size (mm3) 0.54 x 0.07 x 0.03 0.25 x 0.18 x 0.15 0.35 x 0.16 x 0.16 Theta range for data collection (°) 5.768 to 55.108 6.17 to 55.07 6.954 to 55.052 -14 ≤ h ≤ 14, -14 ≤ k ≤ 14, -18 ≤ l ≤ -38 ≤ h ≤ 38, -17 ≤ k ≤ 17, -34 ≤ l ≤ 34 -14 ≤ h ≤ 14, -14 ≤ k ≤ 14, -15 ≤ l ≤ 14 Index ranges 19 Reflections collected 234573 30499 18874 Independent reflections, 5960[R(int) = 0.0475] R(int) 24738[R(int) = 0.0624] 2422 [R(int) = 0.0237] Completeness to theta (%) 99.8 99.8 99.7 Max. and min. transmission 0.5087 and 0.7456 0.5996 and 0.7456 0.4798 and 0.7456 Refinement Method Full-matrix least-squares on F2 Full-matrix least-squares on F2 Full-matrix least-squares on F2 Data/restraints/parameters 24738/2819/1680 5960/6/377 2422/0/132 Goodness-of-fit on F2 1.101 1.168 1.211 Final R indices R1 = 0.0297, wR2 = 0.0517 [I>2sigma(I)] R1 = 0.0472, wR2 = 0.1009 R1 = 0.0129, wR2 = 0.0323

R indices (all data) R1 = 0.0588, wR2 = 0.1070 R1 = 0.0372, wR2 = 0.0543 R1 = 0.0130, wR2 = 0.0323 Largest diff. peak and hole (e.A-3) 2.38 and -1.85 1.28 and -1.37 0.40 and -1.44

Table 3.1 Parameters for X-Ray Structures in Chapter 3

137 Complex Pt(NCN) tetramer C5a C6

Emperical Formula C113.5H179N16O3.5Pt4 C40 H63 Cl Li2 N4 O4 Pt C36H58ClLi2N5O4Pt Formula weight 2604.08 908.36 869.29 Temperature (K) 100 100(2) 100 Wavelength (Å) 0.71073 0.71073 0.71073 Crystal System monoclinic Monoclinic orthorhombic

Space group P21/c C 2/c Pna21 a = 14.1078(12), b = 17.8284(15), c = a = 36.847(4), b = 12.1393(13), c = a = 9.5565(5), b = 26.8313(14), c = Unit cell axes (Å) 46.663(4) 23.040(3) 15.1803(8) Unit cell angles (°) β = 94.074(2)° = 90 β= () γ =  orthorhombic Volume (Å3) 11707.1(17) 8508.4(16) 3892.4(4) Z 4 8 4 Demsity (mg/m3), calc. 1.477 1.418 1.483 Absorption coeff. (mm-1) 4.819 3.403 3.716 F(000) 5248 3712 1768 Crystal size (mm3) 0.16 x 0.12 x 0.05 0.21 x 0.12 x 0.07 0.35 x 0.06 x 0.04 Theta range for data collection (°) 5.79 to 55.142 1.34 to 28.40 5.882 to 55.134 Index ranges -18 ≤ h ≤ 18, -23 ≤ k ≤ 23, -60 ≤ l ≤ 60 -49<=h<=49, -16<=k<=16, -30<=l<=30 -12 ≤ h ≤ 12, -34 ≤ k ≤ 34, -19 ≤ l ≤ 19 Reflections collected 292634 145652 92641 Independent reflections, 26958[R(int) = 0.0972] R(int) 10254 [R(int) = 0.0499] 8963[R(int) = 0.0605] Completeness to theta (%) 99.8 98.5 99.8 Max. and min. transmission 0.6049 and 0.7456 0.7966 and 0.5351 0.5391 and 0.7456 Refinement Method Full-matrix least-squares on F2 Full-matrix least-squares on F2 Full-matrix least-squares on F2 Data/restraints/parameters 26958/342/1412 10254/166/526 8963/25/449 Goodness-of-fit on F2 1.21 1.043 1.265 Final R indices [I>2sigma(I)] R1 = 0.0569, wR2 = 0.0969 R1 = 0.0346, wR2 = 0.0648 R1 = 0.0380, wR2 = 0.0602 R indices (all data) R1 = 0.0790, wR2 = 0.1027 R1 = 0.0480, wR2 = 0.0708 R1 = 0.0457, wR2 = 0.0617 Largest diff. peak and hole (e.A-3) 283 and -3.96 2.572 and -1.952 2.00 and -2.96

Table 3.2 Parameters for X-Ray Structures in Chapter 3

138 Complex C8 C9 C11

Emperical Formula C64.75H75N6O1.25P2Pt C23H34ClN5OPt C37H58Li2N4O4Pt Formula weight 1214.33 627.09 831.84 Temperature (K) 100 100 100 Wavelength (Å) 0.71073 0.71073 0.71073 Crystal System Monoclinic monoclinic Monoclinic

Space group P21/C P21/c P21/C a = 15.1308(6), b = 26.3162(11), c = a = 16.4244(6), b = 7.8124(3), c a = 10.2030(6), b = 24.8268(14), c = Unit cell axes (Å) 15.2971(6) =20.8885(8) 30.4664(17) Unit cell angles (°) β= 106.2490 β =112.4040(10) Volume (Å3) 5847.8(4) 2477.98(16) 7717.3(8) Z 4 4 8 Demsity (mg/m3), calc. 1.379 1.681 1.432 Absorption coeff. (mm-1) 2.501 5.794 3.677 F(000) 2494 1240 3392 Crystal size (mm3) 0.2 x 0.14 x 0.01 0.15 x 0.09 x 0.09 0.15 x 0.09 x 0.05 Theta range for data collection (°) 5.818 to 55.134 5.858 to 55.068 5.812 to 55.214 Index ranges -19 ≤ h ≤ 19, -34 ≤ k ≤ 34, -19 ≤ l ≤ 19 -21 ≤ h ≤ 21, -10 ≤ k ≤ 10, -27 ≤ l ≤ 27 -13 ≤ h ≤ 13, -32 ≤ k ≤ 32, -39 ≤ l ≤ 39 Reflections collected 134940 55644 146793 Independent reflections, 5696[R(int) = 0.0467] 17802[R(int) = 0.0525] R(int) 13493[R(int) = 0.0660] Completeness to theta (%) 99.8 99.9 99.8 Max. and min. transmission 0.6058 and 0.7456 0.6114 and 0.7456 0.6490 to 0.7456 Refinement Method Full-matrix least-squares on F2 Full-matrix least-squares on F2 Full-matrix least-squares on F2 Data/restraints/parameters 13493/577/814 5696/6/327 17802/36/898 Goodness-of-fit on F2 1.106 1.236 1.249 Final R indices R1 = 0.0300, wR2 = 0.0518 R1 = 0.0363, wR2 = 0.0625 [I>2sigma(I)] R1 = 0.0383, wR2 = 0.0672

R indices (all data) R1 = 0.0558, wR2 = 0.0732 R1 = 0.0379, wR2 = 0.0546 R1 = 0.0434, wR2 = 0.0645 Largest diff. peak and hole (e.A-3) 0.82 and -1.19 1.60 and -1.38 2.11 and -2.69

Table 3.3 Parameters for X-Ray Structures in Chapter 3

139 3.5 References to Chapter 3

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A. M. Z.; Cazin, C. S. J.; Nolan, S. P. Organometallics 2017, 36, 2861–2869.

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2000, 19, 1355–1364.

(18) Cook, B. J.; Chen, C.-H.; Pink, M.; Caulton, K. G. Dalt. Trans. 2018, 47, 2052–2060.

(19) Bailey, W. D. Late Transition-Metal Complexes Supported by Pincer Ligands :

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Kemp, R. A.; Giambastiani, G.; Goldberg, K. I. Organometallics 2015, 34, 3998–4010.

(22) Dias, H. V. R.; Diyabalanage, H. V. K.; Eldabaja, M. G.; Elbjeirami, O.; Rawashdeh-

Omary, M. A.; Omary, M. A. J. Am. Chem. Soc. 2005, 127, 7489–7501.

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141 9538–9539.

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(28) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.; Nudelman, A.; Stoltz, B.

M.; Bercaw, J. E.; Goldberg, K. I. Organometallics 2010, 29, 2176–2179.

(29) Brune, H. A.; Stapp, B.; Schmidtberg, G. J. Organomet. Chem. 1986, 301, 129–137.

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143 Chapter 4

Synthesis of Bis(phosphino)amine Ligated PtII Species and Investigations Towards C-H coupling

4.1 Introduction

Many recently reported metal precatalysts for hydrogenation leverage the ability of the ligand to assist in the manipulation of dihydrogen to reduce small molecules.1 Such metal-ligand cooperation (MLC) contrasts with traditional reaction mechanisms which usually invoke oxidation and reduction at the metal center, with the ligand(s) acting as a spectator. An example of MLC is

Noyori’s asymmetric hydrogenation system (first published in 1987)2 where a multifunctional amine ligand promotes hydrogenation.3 Noyori originally proposed a 2 + 2 addition mechanism for hydrogenating ketones (Figure 4.01a). Here, a Ru-H intermediate with a ligand NH2 moiety promoted a hydride transfer and an N-H transfer to a carbonyl by a concerted addition pathway. The starting Ru species could then be regenerated by heterolytic H2 cleavage across a Ru-NH bond.

However, computational work by Gordon revealed a different mechanism (Figure 4.01b). In this case, a hydride is first transferred to the ketone substrate, which is then stabilized through H-bonding.

After, a substrate bridge promotes protonation from a Ru-(H2) moiety, regenerating the starting Ru species. Since then, the concept of small molecule activation by MLC has expanded to include a greater range of substrates and a broader class of ligands.1 144

Figure 4.0 1 Proposed acetophenone hydrogenation by (a) Noyori and (b) Gordon.

One new class of ligands used in MLC reactions are chelating aminophosphine ligands, containing P and N donor atoms. Numerous catalytic reactions including hydroformylation, alkene isomerization, hydration and transfer hydrogenation reactions, among others, have employed these ligands.4 In fact, bidentate aminophosphine (PN) ligated systems can often accomplish more efficient catalysis than their analogous bidentate diphosphine (PP) and diamine (NN) counterparts due in part to their ability to participate in MLC.4 PN ligated aminophosphine complexes are known to exhibit hemilability at the bound N moiety, providing a potential low valent metal with an additional reactive site. This is due to strained chelate rings and a preference for ligation of soft P donors over hard N donors when bound to soft metals (RuII, PtII, PdII, RhIII and others).5–8 For example, a PN ligated Ru complex was shown to facilitate isomerization of 1-pentene by a mechanism invoking MLC (Figure

4.02a).9 By density functional theory (DFT), calculations suggest that the aminophosphine ligand

145

9 helps deprotonate the 1-pentene α-CH2 moiety (a formal C-H oxidative addition). Additionally, a deprotonated bis(diphenylphosphino)amine (dppa) ligand, when bound to Fe, has been reported to heterolytically cleave dihydrogen (an analog to a C-H bond) to form an Fe-H complex with a

1 10 protonated ligand backbone, bound in a κ fashion (Figure 4.02b). Further addition of H2 leads to formation of a Fe(H)2 complex, with loss of ligand. Along with formation of Fe-H and N-H bonds, an additional driving force for this activation was postulated to be relief of strained 4 membered deprotonated chelates. Once protonated, the P-Fe-P bite angle increased by 4-5 degrees, leading to a more stable complex.10 These examples illustrate the ability of coordinated aminophosphines to help cleave strong X-H bonds.

Figure 4.0 2 (a) Proposed mechanism of isomerization of 1-pentene to trans 2-pentene H by MLC. (b) Heterolytic hydrogen cleavage by Fe(*N(P(C6H5)2)2)2( N(P(C6H5)2)2) to H form Fe( N(P(C6H5)2)2)2(H)2 by MLC.

146

While the main goal of this project is geared towards heterolytic cleavage of C-H bonds by

PtII complexes, it is often helpful to look at the microscopic reverse of this process, i.e. elimination

II of the alkane from a Pt -CH3 complex, facilitated by a protic ligand. Indirect evidence for elimination of methane through an MLC type mechanism was observed by the Goldberg group while studying

tBu H a platinum dimethyl di-tert-butylphosphino-2-aminopyridine [Pt( PN N)(CH3)2] complex

(Scheme 4.01).11 Upon thermolysis, methane was formed in the presence of trapping pyridine ligand

Scheme 4.01. Elimination of methane upon thermolysis in the (a) presence of pyridine (b) absence of pyridine

tBu to yield a new deprotonated platinum complex. The formation of Pt( PN*N)(CH3)(pyr-d5) suggested coupling of the acidic N-H proton on the ligand and the methyl group bound to the metal

tBu H center. Interestingly, thermolysis of Pt( PN N)(CH3)2 in C6D6, in the absence of exogeneous

tBu D ligands, led to a significant amount of Pt( PN N)(CH3)(C6D5) being formed, along with methane

(Scheme 4.01b). This suggests that in the absence of a coordinating ligand, C-H bond formation occurred (forming methane), followed by C-H heterolytic cleavage of solvent. However, in the presence of pyridine, an intermediate can be trapped after C-H bond formation occurs, demonstrating a platform for studying the microscopic reverse of C-H activation.

147

Leveraging the ability of the ligand to assist in the activation of strong bonds has led us to investigate similar aminophosphine based ligands. Described below are preliminary investigations of new and previously reported dppa ligated Pt species and their reactivity towards the cleavage or formation of X-H (X = C, H) bonds. The dppa ligand contains a N-H moiety which has been shown to exhibit MLC in dihydrogen cleavage when ligated to Fe, as discussed above.10 The Pt dppa

II 12–14 complexes Pt (dppa)(X)2 (X = CH3, Cl) have been previously reported. The reactivity of this and other aminophosphine ligated PtII complexes were investigated and compared to the reactivity of the similar bidentate phosphinopyridine ligated PtII complex previously studied by the Goldberg group. The complexes all contain a secondary sphere N-H moiety.

4.2 Results and Discussion

II 4.21 Ligation of Protic Amino(bisphosphines) to Pt

Synthesis of aminophosphine and bis(phosphino)amine ligands generally proceed by

15 H condensation reaction mechanisms. While, N(P(C6H5)2)2 is formed from condensation of

H H t (C6H5)2PCl with N(Si(CH3)3), this does not work for the analogous N(P( Bu)2)2 ligand. Instead,

H t t 16 N(P( Bu)2)2 can be synthesized by a self-condensation route of Bu2PCl with NaNH2.

H H t Subsequently, N(P(C6H5)2)2 and N(P( Bu)2)2 were both prepared. In 1970, Tolman quantified that

H t N(P( Bu)2)2 contains a more electronically donating and sterically congested phosphine groups than

H 17 N(P(C6H5)2)2, as evidenced by ⱱCO data and cone angle analysis of their PR3 analogs, respectively.

However, both bis(phosphino)amine ligands are quite sterically crowded and might allow us to exploit disfavored geometries.

148

To probe how the size of the PR2 groups affects coordination, bis(phosphino)amine ligands

II were ligated to Pt X2 to form several complexes (Figure 4.03 and 4.04). More specifically,

H II H 14 N(P(C6H5)2)2 was ligated to Pt to form the previously reported Pt( N(P(C6H5)2)2)(CH3)2 (D1a) ,

H 13 13 [Pt( N(P(C6H5)2)2)2][BF4]2 (D2) , and Pt((*N(P(C6H5)2)2)2 (D3) , (Note: * indicates deprotonated

31 1 1 1 amine) with P{ H} NMR resonances at 29.7 ppm ( JPt-P = 1489 Hz), 22.0 ppm ( JPt-P = 2100 Hz),

H t II H Figure 4.0 3 Metalation of N(PR2)2 (R = C6H5, Bu) with Pt to form Pt( N(P(R)2)(CH3)2 t H t [R = C6H5 (D1a), Bu (D1b)] and Pt( N(P( Bu)2)2)(Cl)2 (D4).

1 and -17.4 ppm ( JPt-P = 1660 Hz), respectively. Additionally, D1a also contains a diagnostic Pt-CH3

2 3 2 resonance at 0.69 ppm ( JPt-H = 75 Hz, JP-H = 12 Hz) and a N-H resonance at 5.65 ppm ( JP-H = 9

3 1 Hz, JPt-H = 63 Hz) in the H NMR spectrum.

H t The analogous N(P( Bu)2)2 complex was synthesized at elevated temperatures (100 °C) to

H t form Pt( N(P( Bu)2)2)(CH3)2 (D1b) and is one of only a handful of examples of transition metal

H t 18,19 ligation by N(P( Bu)2)2 (Figure 4.03). This species exhibited a characteristic Pt-CH3 resonance

3 2 3 at 0.44 ppm ( JP-H = 8 Hz, JPt-H = 74 Hz) and a N-H resonance at 4.43 ppm ( JPt-H = 56 Hz) in the

149

1 31 1 1 H NMR spectrum. The P{ H} NMR spectrum also contained a singlet at 73.3 ppm ( JPt-P = 1501

Hz). Although the spectroscopic resonances and J values are similar to those exhibited by D1a (e.g.

1 14 1 JPt-P = 1475 Hz), the JPt-P of D1b was found to be slightly larger. It has previously been shown

1 using similar bidentate diphosphine ligands (with varying PR2 groups) that larger JPt-P values correlate with wider P-M-P bite angles.17,20 This suggests the P-Pt-P bite angle of D1b would be larger than D1a. While Tolman cone angles do not directly correlate with P-M-P bite angles, a cone analysis is an estimation to compare sterics on different phosphine ligands.21 A similar correlation was made describing the relationship between the small cone angle of PMe3 and the small bite angle

22 of a bidentate P2Pt complex. Obtaining solid state structures of D1a and D1b would confirm this trend, and this is demonstrated for homoleptic complexes later in this chapter.

H t Similarly, addition of N(P( Bu)2)2 to a CD2Cl2 solution of Pt(S(CH3)2)2Cl2 gave one major product by 1H and 31P{1H} NMR spectroscopy. The same product was observed spectroscopically

H t in CD2Cl2 (along with free ligand) when 2 equiv. of N(P( Bu)2)2 and 9 equiv. NEt3 were added to a

1 toluene suspension of Pt(S(CH3)2)2Cl2. The major product contains a larger JPt-P = 3260 Hz to

H 1 13 31 1 Pt( N(P(C6H5)2)2)Cl2 ( JPt-P = 3130 Hz) by P{ H} NMR spectroscopy, suggesting

H t Pt( N(P( Bu)2)2)Cl2 (D4, Figure 4.03), which is also in accordance with the comparison of D1a and

D1b, previously discussed. Additionally, by 1H NMR spectroscopy, the tBu moieties exhibited a

3 3 broad doublet ( JP-H = 16 Hz), which is similar to the Pt(CH3)2 variant ( JP-H = 13 Hz). However, the

3 N-H moiety contains a large JPt-H coupling (210 Hz), which is in stark contrast to D1b ( JPt-H = 56

1 1 Hz) and could suggest it is more represented as a JPt-H interaction, where JPt-H values as high as 180

Hz were observed in zwitterionic PtII complexes with a direct N-H—Pt interaction.23 A similar

150

Figure 4.0 4 Synthetic pathway resulting in formation of Pt(*N(PR2)2)2 [R = C6H5 (D3), t H Bu]. Homoleptic [Pt( N(P(C6H5)2)2)2][BF4] (D2) is an intermediate in the formation of t D3. Thermal Ellipsoid plot of D3 and Pt(*N(P( Bu)2)2)2 at 50 % probability and H-atoms omitted for clarity. Selected bond lengths and angles for D3: Pt(1)-P(1) 2.331(1) Å, Pt(1)- P(3) 2.325(2) Å, P(1)-N(1) 1.654(7) Å, P(3)-N(2) 1.650(4) Å, P(1)-Pt(1)-P(2) 65.00(7) ° t P(3)-Pt(1)-P(4) 64.51(7) ° and Pt(*N(P( Bu)2)2)2: Pt(1)-P(1) 2.383(2) Å, P(1)-N(1) 1.656(6) Å, P(1)-Pt(1)-P(2) 65.52(5) °. interaction could be present in D4, where the N-H moiety would be pointed towards the metal. When

H t metalation with 2 equiv. of N(P( Bu)2)2 to Pt(S(CH3)2)2Cl2 was performed in the presence of 2 equiv.

1 31 1 of NaNH2 at 60 °C in toluene-d8, a mixture of species was observed by both H and P{ H} NMR spectroscopy. Removal of the solvent and redissolution of the reaction mixture in CD2Cl2 revealed a

t 1 31 1 single complex, presumably deprotonated Pt(*N(P( Bu)2)Cl2, evidenced by H and P{ H}

t 31 1 1 spectroscopy. The deprotonated Pt(*N(P( Bu)2)Cl2 contained similar P{ H} and H NMR resonances compared protonated D4 however no N-H resonance was observed. Upon exposure of a

t CD2Cl2 solution of Pt(*N(P( Bu)2)Cl2 to air, reformation of protonated D4 was observed (determined by reappearance of the N-H resonance at 5.22 ppm). It is possible that the homoleptic

H t 2+ Pt( N(P( Bu)2)2)2 cannot be formed, as it was not observed spectroscopically when 2 equiv. of

151

H t N(P( Bu)2)2 was added to a neutral or basic Pt(S(CH3)2)2Cl2 solution. Only the monoligated D4 was

t formed. However, an X-ray quality crystal was obtained of Pt(*N(P( Bu)2)2)2 after a CD2Cl2 solution

t of Pt(*N(P( Bu)2)2)(CH3)2 (D1b) decomposed over the course of several weeks (Figure 4.04).

t Further characterization of Pt(*N(P( Bu)2)2)2 was not performed, although it is noted that this is the

H t first X-ray structure obtained of transition metal bound N(P( Bu)2)2. It is unclear how homoleptic

t Pt(*N(P( Bu)2)2)2 was formed and further investigations are required.

The known Pt(*N(P(C6H5)2)2)2 (D3) was synthesized according to literature procedures and previously unobtained solid-state data was collected (Figure 4.04).13 As with protonated

H 1 31 1 Pt( N(P(C6H5)2)2)2 (D2), deprotonated D3 appeared symmetric in solution by H and P{ H} NMR

t spectroscopy. Pt(*N(P( Bu)2)2)2 is similarly symmetric in the solid-state, noted by the similar Pt-P bonds (2.325(2) Å and 2.331(1) Å) and P-Pt-P bite angles (65.00(7) ° and 64.51(7) °, Figure 4.04).

Complex D3 contains an additional strain which is manifested by a smaller P-Pt-P bite angle (by 5

°) and a slightly longer Pt-P bond (by 0.04 Å), compared to the previously reported X-ray structure

H 2+ 13 of the protonated Pt( N(P(C6H5)2)2)2 (D2, 69.90(7) °, 2.293(2) Å). Clearly, deprotonation of the metalated ligand, which already contains a strained 4 member ring, causes an additional distortion to promote potential increased intramolecular steric contact of the P(C6H5)2 groups. It should be noted that the P-Pt-P bite angles observed in the solid state are unusually small. Out of the large number of bidentate P-Pt-P complexes in the literature (> 1000), only a few are known to have

24–26 t smaller bite angles (< 10). The P-Pt-P bite angle of deprotonated Pt(*N(P( Bu)2)2)2 (65.52(5) °,

t Figure 4.04), which contains a larger P( Bu)2 moiety, is greater than analogous Pt(*N(P(C6H5)2)2)2

(D3, 64.51(7) °, Figure 4.04) in the solid state.

152

4.22 Reactivity of Pt(*N(P(C6H5)2)2)2 (D3) towards X-H Activation

H As Fe( N(P(C6H5)2)(*N(P(C6H5)2)2 was able to heterolytically cleave H2 to form

H Fe( N(P(C6H5)2)2(*N(P(C6H5)2)H due to the strained nature of the deprotonated metal-ligand

10 chelate, Pt(*N(P(C6H5)2)2)2 (D3) was investigated for its ability to cleave X-H (X = C and H) bonds. Heating of D3 in C6D6 resulted in no reaction up to 140 °C. Higher temperatures were not investigated. To investigate activation of a kinetically more accessible X-H substrate, dihydrogen was used. Compound D3 was pressurized with 3 atm. of dihydrogen in C6D6 at room temperature

1 Figure 4.0 5 H NMR spectra of D3 (bottom) and subsequent pressurization with H2 (middle) or addition of H2O (top) and generation of triplet at -0.18 ppm (in middle and top spectra). and a 3-line pattern (with the middle line being slightly larger than the two outside) was recorded at

-0.18 ppm in the 1H NMR spectrum (Figure 4.05). Complex D3 was observed as the only metallic product by 1H and 31P{1H} NMR spectroscopy and ESI-MS. The integration of the 3-line pattern increased over time (compared to ligand signals of D3) which suggests it may not be associated with

D3. Additionally, the same 3-line resonance was observed with the addition of 2 equiv. H2O (no H2

153

1 present, Figure 4.05) and disappeared upon addition of D2O by H NMR spectroscopy. A potential explanation is H being split by another spin active (I = 1) nucleus, possibly 14N or 2H.27,28 Performing a 1H{14N} and/or a 2H NMR experiment will elucidate if this triplet is the result of N-H or H-D splitting. However, another explanation is the 3-line resonance is a singlet with a doublet additionally centered at -0.18 ppm. Performing decoupling experiments with other spin active (I = ½) nuclei, (e.g.

1H{31P} or 1H{195Pt}) could elucidate if this is the source of the 3-line resonance.

After pressurizing a D3 suspension with 3 atm. dihydrogen in C6D6, the reaction was heated by gradually raising the temperature and reactivity was not observed until 100 °C. The reaction mixture was subsequently heated at 100 °C for 21 hours and the disappearance of starting material was accompanied by the appearance of multiple broad aromatic resonances with no associated hydride resonances in the 1H NMR spectrum. A single resonance was observed in the 31P{1H} NMR

H spectrum at 13.28 ppm, which did not correspond to free protonated ligand N(P(C6H5)2)2 (although

195 *N(P(C6H5)2)2 ligand was never synthesized to compare). No Pt splitting was observed; however, this could be due to poor signal to noise resulting from low solubility in benzene. Aryl activation of

P-C6H5 and/or ligand degradation is possible, yet undetermined. Reactivity in more solubilizing solvents will allow for more facile spectroscopic monitoring. Additionally, further characterization of this reaction mixture by GC-MS and ESI-MS could elucidate if ligand degradation is occurring or if another Pt complex is forming.

H H t 4.23 Towards C-H coupling from Pt( N(P(C6H5)2)2(CH3)2 (D1a) and Pt( N(P( Bu)2)2(CH3)2

(D1b)

With two similar Pt(CH3)2 complexes in hand (D1a and D1b), thermolysis was explored.

The goal was to observe if C-H coupling between the Pt-CH3 ligand and the N-H moiety is possible.

154

1 31 1 Complex D1b was heated in C6D6 for 15 hrs. at 140 °C; no reaction was noted by H or P{ H}

NMR spectroscopy and higher temperatures were not explored. Proton shuttles have been used in a variety of systems and have even been shown computationally to significantly lower the activation barrier for reactions that involve proton transfer.1 Upon thermolysis of D1b in the presence of a potential proton shuttle, namely water (150 equiv.), a reaction occurred at 120 °C in C6D6. After two hours, a new, symmetric complex had cleanly formed. This complex exhibited a singlet in the

31 1 1 P{ H} NMR spectrum with a large JPt-P splitting of 3860 Hz (Figure 4.15b). Coupling of this magnitude is highly unusual for PtII species and could indicate a reduction to Pt0.29 Another explanation would be formation of a dinuclear species with each ligand bridging between the two

1 metals and relieving the strain from the small bite angle of the ligand. Similar JPt-P to that of this unidentified product have been observed for such dinuclear species.13,30 In the 1H NMR spectrum, a highly deshielded singlet at 17 ppm and a singlet at -0.5 ppm (neither with observable 195Pt coupling) were also observed with a relative 1:2 integration (Figure 4.15a). The former is perhaps indicative of a proton involved in hydrogen-bonding and the latter perhaps belonging to a hydroxide ligand.

Strangely, no Pt-CH3 resonances were observed in C6D6 or CD2Cl2, although it could be hidden

t under the broad Bu resonances around 1.28 ppm. Dissolution of the thermolysis product in C6H6 revealed no 2H resonances. Examination by ESI-MS in a MeOH/iPrOH mixture generated peaks

t i + with an m/z of 1122.2 (potentially [Pt(µ-*N(P( Bu)2)2)(CH3)(OH)]2( PrOH) ) and 565.2 (potentially

1 H t + [Pt(κ - N(P( Bu)2)2)(CH3)(OH)(MeOH)] ), additionally suggesting a dinuclear complex. Platinum complexes with sterically encumbered phosphine and bis(phosphino)amine ligand architectures often exhibit an “A frame” architecture,14,31 so this dinuclear Pt complex with bulky tBu ligands could show this geometry. The exact identity of this complex is still currently unknown; however, no observance of methane (or ethane) elimination suggests that water is promoting a different

155 reaction other than C-H coupling (reverse of C-H activation). Additional proton shuttles, such as alcohols, could be investigated to see if water is unique in its reactivity with D1b. Other proton shuttles might not exhibit this reactivity and the desired C-H coupling might be observed.

Additionally, a solid-state structure would help identity the thermolysis product.

H As thermolysis of D1b did not form a C-H coupled product, Pt( N(P(C6H5)2)2)(CH3)2 (D1a)

1 was next investigated. Thermolysis of D1a in C6D6 for 48 hrs. at 100 °C was monitored by H and

31P{1H} NMR spectroscopy until complete disappearance of starting material was observed.

1 31 1 Although no Pt product was observed by H or P{ H} NMR spectroscopy, gratifyingly, CH4 (C-H coupled product) was observed (Figure 4.06a). When the N-D variant of D1a

D (Pt( N(P(C6H5)2)2)(CH3)2, generated by stirring D1a in a D2O/THF mixture and confirmed by

32 comparison to similar complexes) was thermolyzed in C6D6, a mixture of CH4 and CH3D was

11 observed. Formation of CH3D likely occurs through N-D/Pt-CH3 coupling and CH4 either forms

IV through an H-D exchange pathway (through a transient Pt -(D)(CH3)) or through some other unknown pathway. Examination of the product mixture by ESI-MS in CH3CN (in air) revealed a

H + main peak at m/z = 1191 (potentially corresponding to [Pt( N(P(C6H5)2)2)(CH3)]2 ). As the thermolysis product is not diamagnetic, the Evans Method test was used to gain additional data.33

The Evans Method revealed a shift in an internal standard (hexamethylbenzene) by 3.26 Hz

156

(corresponding to µeff = 0.97, assuming a M.W. of 1191) under N2, and 13.70 Hz (corresponding to

µeff = 2.0, assuming a M.W. of 1191) in air. Evans’ method experiments suggest different paramagnetic solutions under inert atmosphere and in the presence of oxygen. Performing an EPR

Figure 4.0 6 (a) Thermolysis of D1a in C6D6 to form a paramagnetic species. (b) Addition H of HBF4 etherate to D1a in pyridine-d5 to form [Pt( N(P(C6H5)2)2)(CH3)(pyr-d5)][BF4] (D5) and subsequent thermolysis to generate ethane. (c) Thermolysis of D1a in C6D6 with 2 equiv. P(C6H5)3 to form Pt(P(C6H5)3)2(CH3)2 and D3. (d) Thermolysis of D1a to form Pt2(µ-*N(P(C6H5)2)2)(µ-CH2)(pyr)2 (D6) and a paramagnetic product. study could help elucidate whether these radical species are metal based or ligand based. Obtaining a solid-state structure would additionally reveal the structure of the thermolysis product.

157

To further elucidate how methane could be forming, reaction of D1a with 1 equiv. of HBF4

H etherate in pyridine-d5 generated a new species, proposed to be [Pt( N(P(C6H5)2)2)(CH3)(pyr-

1 d5)][BF4] (D5, Figure 4.06b 88 % spectroscopic yield), along with methane. Examination of the H

3 NMR spectrum of D5 revealed a downfield N-H resonance at 10.59 ppm ( JH-Pt = 126 Hz) and a

2 3 3 methyl ligand at 0.97 ppm (dd, JPt-H = 57 Hz, JH(trans)-P = 7.3 Hz, JH(cis)-P = ~2 Hz*). (*Note – the

3 broad doublet of doublets prevents accurate assignment of JH(cis)-P and additional NMR experiments

31 1 2 are required). Examination of the P{ H} NMR spectrum revealed two doublets at 11.9 ppm ( JP-P

1 2 1 = 39 Hz, JPt-P = 3476 Hz) and 36.5 ppm ( JP-P = 39 Hz, JPt-P = 1340 Hz). The NMR shifts and J

+ 34 values are similar to a previously reported cationic diphosphine ligated Pt-(pyr)(CH3) complex.

Interestingly, thermolysis of the proposed monomethyl species D5 at 100 °C did not eliminate methane and instead generated ethane, along with no resonances in either the 1H NMR or

31P{1H} NMR spectra. A small amount of yellow solid was observed at the bottom of the J. Young

31 1 NMR tube. Redissolution of the solid in CD3CN revealed a single main resonance by P{ H} with no 195Pt coupling (which could be due to poor signal to noise resulting from the very small amount of solid). Additionally, the solid contained many aromatic resonances in the 1H NMR spectrum and

2 3 3 a Pt-CH3 resonance at 0.65 ppm ( JPt-H = 61 Hz, Jtrans,P-H = 7.9 Hz, Jcis,P-H = 2.5 Hz). It is unclear what this Pt-CH3 species is (which contains a ligated phosphine) and could be unreacted starting material due to similar coupling constants of D5 in pyr-d5. It is possible that an additional paramagnetic species was formed, however, additional experiments are required to assign the product species (i.e. an Evan’s Method test, EPR experiments, ESI-MS and/or a solid-state structure). Liberation of ethane over methane in C6D6 is contrary to previous examples of cationic

+ bidentate diphosphine Pt-(CH3)(L) reactivity in the literature (which typically generate methane

158 after C-H activation of solvent).35 There is precedent for C-C bond forming reactions from PdII-Ar complexes.36

To try and trap an intermediate in the formation of the unknown paramagnetic compound

H when Pt( N(P(C6H5)2)2)(CH3)2 (D1a) was thermolyzed in C6D6, D1a was again was thermolyzed at

Figure 4.0 7 Thermal ellipsoid plot of D6 at 50 % probability and H-atoms omitted for clarity. Right orientation highlights “A-Frame”. Selected bond lengths and angles for D6: Pt(1)-P(1) 2.299(1) Å, Pt(1)-C(59) 2.070(4) Å, Pt(1)-N(3) 2.153(4) Å, Pt(1)-Pt(2) 2.9701(6) Å.

100 °C in the presence of 2 equiv. of P(C6H5)3 in C6D6. This cleanly formed the known

37 Pt(P(C6H5)3)2(CH3)2 and Pt(*N(P(C6H5)2)2)2 (D3), with concomitant release of CH4 (Figure 4.06c).

Presence of a strongly donating ligand, P(C6H5)3, does not appear to trap an intermediate and instead forms the homoleptic D3 as the only *N(P(C6H5)2)2 containing product. Pyridine, a weaker binding ligand than P(C6H5)3, which could additionally act as a base, was next used. It should be noted that

1 pyridine does not deprotonate the N-H moiety; D1a is stable for at least 1 hour in pyridine-d5 by H

NMR spectroscopy, although longer times were not explored. Thermolysis of D1a with an excess

1 31 1 of pyridine (50 equiv.) in C6D6 liberated CH4 and a single diamagnetic product by H and P{ H}

NMR spectroscopy (15 % spectroscopic yield). Up to 20 % spectroscopic yield of the diamagnetic

159 product can be obtained in neat pyridine-d5. A solid state structure of the diamagnetic product was additionally obtained and indeed revealed an “A-Frame” type structure, Pt2(µ-*N(P(C6H5)2)2)(µ-

CH2)(pyr-d5)2 (D6, Figure 4.06 and 4.07). Pt-P bonds lengths (2.299(1) Å) in deprotonated D6 are slightly shorter than the deprotonated homoleptic D1a and D1b analogous bonds (2.331(1) Å and

2.383(2) Å, respectively) and demonstrate the relief in ring strain obtained in “A-Frame” type structures from 4 membered ring small bite angle complexes. Formation of Pt methylene dimers are known, yet only a handful of solid-state structures have been obtained.30,38 Reported Pt methylene dimers are often synthesized with reagents such as diazomethane and formation from thermolysis reactions of Pt-(CH3)2 complexes have not been reported. Interestingly, formation of a methylene bridged Pt complex was obtained from deprotonation of a cyclometalated 1,2-

39 Bis(diphenylphosphino)ethane (dppe) ligated [Pt-CH3][I] complex with NaOMe, and basic pyridine could be acting in a similar manner to NaOMe under thermolysis conditions. Evaluating thermolysis of D6 with NaOMe addition could test this hypothesis and could form a diamagnetic analog to D6, or potentially Pt2(µ-*N(P(C6H5)2)2)(µ-CH2)(OMe)2. Formation of this Pt-OMe complex could then indicate a similar mechanism is at play to the previously reported dppe ligated

Pt methylene dimer. Complex D6 exhibited a singlet by 31P{1H} NMR spectroscopy with a complex

P-Pt AA’A’’A’’’XX’ spin system (Figure 4.14b). While the exact JPt-P and JP-P values were not obtained and require additional 2D NMR experiments, similar 31P{1H} NMR spectra have been observed in similar “A-Frame” type structures.40 As D6 was formed in only 15 % yield in a

C6D6/pyridine mixture, the remaining 85% could be accounted for by ligand decomposition, and/or a paramagnetic Pt complex. An Evan’s Method test was not performed to determine if paramagnetic species were present. Examination of the product mixture by ESI-MS in CH3CN in air revealed two

H + peaks at m/z = 636.2 and 679.2 (potentially corresponding to [Pt( N(P(C6H5)2)2)(CH3)(NCCH3)]

160

H + and [Pt( N(P(C6H5)2)2)(CH3)(pyr-d5)] , respectively). D6 does not appear to be stable to ESI-MS conditions.

161

4.3 Conclusion

H H t II In conclusion, bidentate N(P(C6H5)2)2 and N(P( Bu)2)2 were ligated to Pt to form complexes that were investigated for their ability to undergo X-H activation (X = C, H) and C-H

H t elimination reactions through an MLC-type mechanism. Synthesis of Pt( N(P( Bu)2)2)(X)2 [X = Cl,

H D4, X = CH3, D1b] and known Pt(*N(P(C6H5)2)2)2 (D3) and Pt( N(P(C6H5)2)2)(CH3)2 (D1a) was

t achieved. Although a viable synthesis of homoleptic Pt(*N(P( Bu)2)2)2 was not accomplished in solution, a small amount was observed to form in the solid state. While D3, a complex prepared to be studied for C-H activation, did not react with C6D6 solvent up to 140 °C, D1b appeared to react with dihydrogen. An unknown species was formed, evidenced by a single resonance by 31P{1H}

NMR spectroscopy and by the appearance of multiple new 1H aromatic resonances, with no observable Pt-H resonance. Aiming to study the microscopic reverse of C-H activation using Pt-CH3 species with protonated backbones, D1a and D1b were investigated. Surprisingly, a thermolysis reaction did not occur for D1b in C6D6 upon heating to 140 °C, yet upon the addition of water, a new

1 Pt-OH species was generated at 120 °C with no methane loss or Pt-CH3 resonance by H NMR spectroscopy. Additionally, examination of the reaction mixture by ESI-MS suggests a dimeric species could have been formed. Absence of methane in the 1H NMR spectrum indicates that water might not be acting as a proton shuttle and is instead reacting with D1b. Conversely, thermolysis of

D1a in C6D6 occurred at 100 °C (with or without water present) to generate methane and a paramagnetic compound (determined by Evan’s Method analysis), potentially a binuclear complex by ESI-MS. Additionally, thermolysis of the N-D variant of D1a formed a mixture of CH4/CH3D, indicating cooperation of the N-D moiety with Pt-CH3. Thermolysis of D1a in a C6D6/pyridine mixture additionally led to methane liberation and a new diamagnetic “A-Frame” complex (D6) in low yield. While the origin of the trapped diamagnetic D6 species is not known at this point, it could

162 be due to presence of a base and additional bases should be explored. In all, generation of methane

(C-H coupling) was achieved. Deuteration experiments suggest some of the formed methane is due to cooperation of the N-H moiety and Pt-CH3 moiety, although additional experiments are necessary to determine how all the generated methane was formed.

4.4 Experimental

4.41 General Experimental

All manipulations were carried out under nitrogen atmosphere using standard Schlenk and glovebox techniques unless otherwise noted. Deuterated solvents were purchased from Cambridge

Isotope Laboratories. Dry tetrahydrofuran, benzene, toluene, pentane, methylene chloride and

41 diethyl ether were obtained by means of a Grubbs-type solvent purification system. C6D6 was dried over sodium/benzophenone ketyl and were vacuum transferred prior to use. CD3CN and pyridine-d5 were dried over activated 3 Å and 4 Å, respectively, molecular sieves. MeOD was dried over dematiaceous earth and vacuum transferred prior to use. CD2Cl2 was dried over calcium hydride and vacuum transferred prior to use. PtCl2(S(CH3)2)2 and Pt(COD)(CH3)2 were synthesized following literature preparations.42 All NMR spectra were obtained on a Bruker

Avance 200 MHz, Bruker Avance 300 MHz or Bruker Avance 500 MHz instrument. The spectra were recorded at 300 K. Chemical shifts are reported in units of parts per million (ppm) downfield of TMS and referenced against proteo-solvent residual resonances (1H) and characteristic solvent

13 31 1 resonances ( C). P{ H} NMR spectra were referenced externally to H3PO4 (85%, 0 ppm). NMR tubes fitted with a J-Young style Teflon valve were used to obtain inert atmosphere NMR data.

Nominal mass accuracy ESI-MS data were obtained by use of a Waters Acquity UPLC system

163 equipped with a Waters TUV detector (254 nm) and a Waters SQD single quadrupole mass analyzer with electrospray ionization.

4.42 Synthesis, Characterization and Spectroscopic Data

H Pt( N(P(C6H5)2)2)(CH3)2 (D1a)

H A 50 mL Schlenk tube was charged with 148.2 mg (0.385 mmol) N(P(C6H5)2)2, 128.2 mg (0.385 mmol) Pt(COD)(CH3)2 and 10 mL THF. The vessel was sealed with a Teflon pin and heated to 60

°C for 6.5 hours. The yellow solution was cooled to room temperature and concentrated to 5 mL.

The solution was layered with pentane and placed into a -30 °C freezer for 12 hours. Colorless crystals formed from the solution, which were collected by filtration, washed with pentane (3 x 1 mL) and dried under reduced pressure (211.6 mg, 90.0 % yield). D1a is a known compound and

1H and 31P{1H} NMR spectroscopy agrees with the previously reported compound.14 1H NMR

3 (CD2Cl2, 200 MHz): δ 7.72-7.27 (20H, m, Ar-H), 5.64 (1H, s, N-H, JPt-H = 65 Hz), 0.68 (6H, t,

3 3 31 1 3 Ar-H, JH-P = 12 Hz, JPt-H = 75 Hz). P{ H} (CD3CN, 162 Hz): δ 29.73 ( JPt-H = 1475 Hz).

164

(a)

(b)

1 31 1 Figure 4.08a. H NMR spectrum (200 MHz) of D1a in CD2Cl2. (b) P{ H} NMR spectrum (81

MHz) of D1a in C6D6.

165

H t Pt( N(P( Bu)2)2)(CH3)2 (D1b)

H t A 50 mL Schlenk tube was charged with 65.0 mg (0.206 mmol) N(P( Bu)2)2, 68.7 mg (0.206 mmol) Pt(COD)(CH3)2 and 10 mL toluene. The vessel was sealed with a Teflon pin and heated to

100 °C for 20 hours. The yellow solution was cooled to room temperature and the volatiles were removed. The yellow mixture was washed with pentane (3 x 3 mL) and the resulting solid collected

1 and dried under reduced pressure (32.2 mg, 29.3 % yield). H NMR (CD2Cl2, 300 MHz): δ 4.43

3 t 3 3 3 (1H, s, N-H, JPt-H = 56 Hz), 1.35 (36H, d, Bu, JH-P = 13 Hz), 0.44 (6H, t, Ar-H, JH-P = 8 Hz, JPt-

31 1 3 H = 74 Hz). P{ H} (CD3CN, 162 Hz): δ 73.25 ( JPt-H = 1501 Hz).

(a)

166

(b)

1 31 1 Figure 4.09a. H NMR spectrum (300 MHz) of D1b in CD2Cl2. (b) P{ H} NMR spectrum (81

MHz) of D1b in C6D6.

H [Pt( N(P(C6H5)2)2)2][BF4]2 (D2)

H A 50 mL round bottom was charged with 155 mg (0.402 mmol) N(P(C6H5)2)2, 78.6 mg (0.201 mmol) Pt(S(CH3)2)(Cl)2, 44.1 mg (0.402 mmol) NaBF4, 10 mL CH2Cl2 and 5 mL MeOH. The solution was stirred for 15 mins and the volatiles were subsequently removed to yield a white solid.

The product was extracted with CH2Cl2 (15 mL) and run through a fritted funnel. The filtrate’s volatiles were subsequently removed under reduced pressure and the solid was washed with diethyl ether (2 x 10 mL) to yield a white solid (75.8 mg, 33.1 %).1H NMR (MeOD, 300 MHz): δ 7.67-

31 1 1 7.19 (40H, m, Ar-H). P{ H} (CD3CN, 162 Hz): δ 5.20 (br s, no JPt-H observed). Broadness is believed to be due to exchange of the N-H moiety, possibly with H2O..

167

(a)

(b)

Figure 4.10a. 1H NMR spectrum (200 MHz) of D2 in MeOD. (b) 31P{1H} NMR spectrum (81

31 1 MHz) of D2 in DMF-h6. Unknown compound observed in P{ H} NMR spectrum at 40.01 ppm.

Pt(*N(P(C6H5)2)2)2 (D3)

168

A 20 mL scintillation vial was charged with 64.5 mg (0.0566 mmol) D2 and 5 mL THF to yield a suspension. While vigorously stirring, 4.5 mg (0.0113 mmol) NaNH2 was added and allowed to stir for 2 hours. The solution was cooled and filtered via a fritted funnel and washed with cold THF

(1 x 3 mL). The solid was collected and dried under reduced pressure (49 mg, 88 %).1H NMR

31 1 1 (CD2Cl2, 300 MHz): δ 7.60-7.11 (40H, m, Ar-H) P{ H} (CD3CN, 162 Hz): δ -17.67 ( JPt-H =

1660 Hz).

(a)

169

(b)

1 31 1 Figure 4.11a. H NMR spectrum (200 MHz) of D3 in CD2Cl2. (b) P{ H} NMR spectrum (81

MHz) of D3 in CD2Cl2

H t Pt( N(P( Bu)2)2)(Cl)2 (D4)

H t A 50 mL Schlenk flask was charged with 47.2 mg (0.155 mmol) N(P( Bu)2)2, 30.2 mg (0.0773 mmol) Pt(S(CH3)2)2(Cl)2 and toluene (10 mL). A reflux condenser was attached under a positive

N2 flow and the vessel was heated to reflux for 15 hours to form a yellow solid. The mixture was filtered via a fritted funnel and the solid was extracted from the fritted funnel with CH2Cl2 (5 mL).

The volatiles were removed under reduced pressure and a sample of the solid was added to a J.

1 Young NMR tube and the following NMR spectra were recorded. H NMR (CD2Cl2, 200 MHz):

170

1 t 3 31 1 δ 5.97 (1H, s, N-H, JPt-H = 210 Hz), 1.51 (36H, d, Bu, JH-P = 16 Hz). P{ H} (CD3CN, 162 Hz):

3 δ 73.25 ( JPt-H = 3260 Hz).

(a)

(b)

171

1 31 1 Figure 4.12a. H NMR spectrum (200 MHz) of D4 in CD3CN. (b) P{ H} NMR spectrum (81

MHz) of D4 in CD3CN.

H [Pt( N(P(C6H5)2)2)(CH3)(pyr-d5)][BF4] (D5)

A J. Young NMR tube was charged with 6.7 mg (0.011 mmol) D1a and pyridine-d5. 1.5 µL of

HBF4 etherate (0.011 mmol, 54 % in diethyl ether) was added and the following NMR spectra

1 3 were recorded. H NMR (pyridine-d5, 400 MHz): δ 9.55 (1H, s, N-H, JPt-H = 126 Hz), 7.99-7.87

2 3 (8H, m, Ar-H), 7.38-7.28 (12H, m, Ar-H), 1.45 ppm (6H, t, Pt-CH3, JPt-H = 57 Hz, JH(trans)-P = 7.3

3 31 1 2 1 Hz, JH(cis)-P = ~2 Hz). P{ H} (CD3CN, 162 Hz): δ 36.54 (d, JP-P = 39 Hz, JPt-H = 1340 Hz),

2 1 11.88 (d, JP-P = 39 Hz, JPt-H = 3476 Hz).

172

(a)

(b)

1 31 1 Figure 4.13. H NMR spectrum (200 MHz) of D5 in pyridine-d5. (b) P{ H} NMR spectrum (81

MHz) of D5 in pyridine-d5.

173

Pt2(µ-*N(P(C6H5)2)2)(µ-CH2)(pyr)2 (D6)

A J. Young NMR tube was charged with 6.7 mg (0.011 mmol) D1a, C6D6 (500 µL) and pyridine- d5 (50 µL). The NMR tube was sealed, heated to 100 °C for 2 days, and cooled to room

1 temperature. The following NMR spectra were recorded. H NMR (C6D6, 400 MHz): δ 8.65-8.54

(8H, m, Ar-H), 8.10-8.06 (1H, m, Ar-H), 7.81-7.71 (1H, m, Ar-H), 7.59-7.49 (9H, m, Ar-H), 7.32-

31 1 2 3 1 6.68 (21H, m, Ar-H), 3.11 (2H, m, Pt-CH2). P{ H} (CD3CN, 162 Hz): δ 42.83 (s, JP-P, JP-P, JP-

2 Pt, and JP-Pt could not be determined and require additional 2D experiments).

(a)

174

(b)

Figure 4.14. 1H NMR spectrum (400 MHz) of impure D6. NMR spectrum obtained from reaction

31 1 mixture of the thermolysis of 1a in C6D6 with pyridine-d5 addition. (b) P{ H} NMR spectrum

(162 MHz) of D6 in C6D6.

Thermolysis product of D1b in C6D6/H2O mixture (Section 4.23)

A J. Young tube was charged with 2 mg (0.004 mmol) D1b, 10 µL (0.56 mmol) H2O and 0.4 mL

C6D6. The NMR tube was heated in an oil bath at 120 °C for 2 hours. The NMR tube was cooled

1 and spectroscopic data was obtained and is shown below. H NMR (CD2Cl2, 200 MHz): δ 15.43

3 (1H, s, N-H), 1.27 (72H, br d, JP-H = 13.1 Hz), -0.83 (2H, s, OH), 7.59-7.49 (9H, m, Ar-H), 7.32-

31 1 6.68 (21H, m, Ar-H), 3.11 (2H, m, Pt-CH2). P{ H} (CD2Cl2, 162 Hz): δ 42.83 ppm

175 a)

1 Figure 4.15. H NMR spectrum (200 MHz) in CD2Cl2 of thermolysis product of D1b in C6D6/H2O

31 1 mixture. (b) P{ H} NMR spectrum (162 MHz) in CD2Cl2 of thermolysis product of D1b in

C6D6/H2O mixture.

4.43 X-ray Crystallography General Information

t Pt(*N(P( Bu)2)2)2 complex: X-ray intensity data were collected at -173 °C on a Bruker APEX II single crystal X-ray diffractometer, Mo-radiation. The data was integrated and scaled using

SAINT, SADABS within the APEX2 software package by Bruker.43 Solution by direct methods

176

(SHELXS, SIR97)44,45 produced a complete heavy atom phasing model consistent with the proposed structure. The structure was completed by difference Fourier synthesis with

SHELXL97.46,47 Scattering factors are from Waasmair and Kirfel.48 Hydrogen atoms were placed in geometrically idealised positions and constrained to ride on their parent atoms with C---H distances in the range 0.95-1.00 Angstrom. Isotropic thermal parameters Ueq were fixed such that they were 1.2Ueq of their parent atom Ueq for CH's and 1.5Ueq of their parent atom Ueq in case of methyl groups. All non-hydrogen atoms were refined anisotropically by full-matrix least-squares.

Even though twinning was resolved, the molecule appeared disordered via lattice symmetry operation –x, y, -z in junction with inversion symmetry. Because of the massive disorder, many restraints were required to stabilize displacement parameters and ensure meaningful t-butyl geometry.

D3 and D6: X-ray intensity data were collected on a Bruker APEXIII D8QUEST49 CMOS area detector, both employing graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) at 100(1) K.

Preliminary in indexing was performed from a series of twenty-four 0.5° rotation frames with exposures of 10 seconds. Rotation frames were integrated using SAINT50, producing a listing of unaveraged F2 and σ(F2) values. The intensity data were corrected for Lorentz and polarization effects and for absorption using SADABS.51 The structure was solved by direct methods –

ShelXT.52 Refinement was by full-matrix least squares based on F2 using SHELXL-2018.53 All reflections were used during refinement. Non-hydrogen atoms were refined anisotropically and hydrogen atoms were refined using a riding model.

177

Table 4.01 Parameters for X-ray Structures in Chapter 4 t Complex Pt(*N(P( Bu)2)2)2 3 6

Emperical Formula C32H72N2P4Pt C48H40N2P4Pt C59H52N4P4Pt2 Formula weight 803.88 963.79 1331.1 Temperature (K) 100(2) 100 100 Wavelength (Å) 0.71073 0.71073 0.71073 Crystal System Monoclinic monoclinic monoclinic

Space group C 2/m Cc P21/c a = 18.231(3), b = 12.484(2), c = a = 23.8432(11), b = 12.4021(6), c = a =10.1832(4), b = 23.6934(6), c = Unit cell axes (Å) 8.8708(15) 17.6773(8) 21.4033(8) Unit cell angles (°) α = 90, β = 115.713(6), γ = 90 β = 130.2320(10) β = 93.7200(10) Volume (Å3) 1819.1(5) 3990.7(3) 5153.2(3) Z 2 4 4 Demsity (mg/m3), calc. 1.468 1.604 1.716

Absorption coeff. (mm-1) 4.056 3.714 5.591 F(000) 832 1920 2600 Crystal size (mm3) 0.130 x 0.070 x 0.070 0.15 x 0.14 x 0.08 0.21 x 0.08 x 0.06 Theta range for data collection 2.049 to 28.404 5.8 to 55.1 5.964 to 55.21 (°) Index ranges -24<=h<=24, -16<=k<=0, -11<=l<=11 -30 ≤ h ≤ 30, -16 ≤ k ≤ 16, -22 ≤ l ≤ 22 -13 ≤ h ≤ 13, -30 ≤ k ≤ 30, -27 ≤ l ≤ 27 Reflections collected 4501 47747 146293 Independent reflections, 2350 [R(int) = 0.0580] 9150[R(int) = 0.0296] 11915[R(int) = 0.0936] R(int) Completeness to theta (%) 99 99.9 99.8 Max. and min. transmission 0.745688 and 0.590347 0.6463 and 0.7456 0.5720 and 0.7456 Refinement Method Full-matrix least-squares on F2 Full-matrix least-squares on F2 Full-matrix least-squares on F2 Data/restraints/parameters 2350/165/158 9150/2/497 11915/0/622

178

Goodness-of-fit on F2 1.006 1.231 1.105 Final R indices [I>2sigma(I)] R1 = 0.0374, wR2 = 0.0899 R1 = 0.0231, wR2 = 0.0435 R1 = 0.0394, wR2 = 0.0586 R indices (all data) R1 = 0.0375, wR2 = 0.0899 R1 = 0.0248, wR2 = 0.0439 R1 = 0.0627, wR2 = 0.0636 Largest diff. peak and hole (e.A-3) 2.305 and -1.338 0.54 and -1.17 1.49 and -2.17

179

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Vita

Braden A. Zahora was born in 1992 to parents Andy and Ingrid Zahora. He grew up in

Charlotte, NC where he graduated high school from Hickory Ridge High School in 2010. He attended the University of North Carolina in Chapel Hill, NC, where he graduated with a

Bachelor of Science degree in chemistry in 2014. While at UNC-CH, Braden worked in the research laboratory of Prof. Jillian L. Dempsey on synthesizing a library of quinoline bases for future proton coupled electron transfer reactions. Braden then attended graduate school at the

University of Washington in Seattle, WA. There, he worked with Prof. Karen I. Goldberg on synthesizing platinum complexes for C-H activation reactions. He obtained a Doctor of

Philosophy degree in inorganic chemistry in 2019.

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